/* ===========================================================
 * JFreeChart : a free chart library for the Java(tm) platform
 * ===========================================================
 *
 * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
 *
 * Project Info:  http://www.jfree.org/jfreechart/index.html
 *
 * This library is free software; you can redistribute it and/or modify it 
 * under the terms of the GNU Lesser General Public License as published by 
 * the Free Software Foundation; either version 2.1 of the License, or 
 * (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful, but 
 * WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY 
 * or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public 
 * License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public License 
 * along with this library; if not, write to the Free Software Foundation, 
 * Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
 *
 * [Java is a trademark or registered trademark of Sun Microsystems, Inc. 
 * in the United States and other countries.]
 *
 * -------------------------
 * XYDifferenceRenderer.java
 * -------------------------
 * (C) Copyright 2003-2005, by Object Refinery Limited.
 *
 * Original Author:  David Gilbert (for Object Refinery Limited);
 * Contributor(s):   Christian W. Zuckschwerdt;
 *
 * $Id: XYDifferenceRenderer.java,v 1.6 2005/02/22 12:49:12 mungady Exp $
 *
 * Changes:
 * --------
 * 30-Apr-2003 : Version 1 (DG);
 * 30-Jul-2003 : Modified entity constructor (CZ);
 * 20-Aug-2003 : Implemented Cloneable and PublicCloneable (DG);
 * 16-Sep-2003 : Changed ChartRenderingInfo --> PlotRenderingInfo (DG);
 * 09-Feb-2004 : Updated to support horizontal plot orientation (DG);
 * 10-Feb-2004 : Added default constructor, setter methods and updated 
 *               Javadocs (DG);
 * 25-Feb-2004 : Replaced CrosshairInfo with CrosshairState (DG);
 * 30-Mar-2004 : Fixed bug in getNegativePaint() method (DG);
 * 15-Jul-2004 : Switched getX() with getXValue() and getY() with 
 *               getYValue() (DG);
 * 25-Aug-2004 : Fixed a bug preventing the use of crosshairs (DG);
 * 11-Nov-2004 : Now uses ShapeUtilities to translate shapes (DG);
 * 19-Jan-2005 : Now accesses only primitive values from dataset (DG);
 * 22-Feb-2005 : Override getLegendItem(int, int) to return "line" items (DG);
 * 
 */

package org.jfree.chart.renderer.xy;

import java.awt.Color;
import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.Shape;
import java.awt.Stroke;
import java.awt.geom.GeneralPath;
import java.awt.geom.Line2D;
import java.awt.geom.Rectangle2D;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

import org.jfree.chart.LegendItem;
import org.jfree.chart.axis.ValueAxis;
import org.jfree.chart.entity.EntityCollection;
import org.jfree.chart.entity.XYItemEntity;
import org.jfree.chart.event.RendererChangeEvent;
import org.jfree.chart.labels.XYToolTipGenerator;
import org.jfree.chart.plot.CrosshairState;
import org.jfree.chart.plot.PlotOrientation;
import org.jfree.chart.plot.PlotRenderingInfo;
import org.jfree.chart.plot.XYPlot;
import org.jfree.data.xy.XYDataset;
import org.jfree.io.SerialUtilities;
import org.jfree.ui.RectangleEdge;
import org.jfree.util.PublicCloneable;
import org.jfree.util.ShapeUtilities;

/**
 * A renderer for an {@link XYPlot} that highlights the differences between two
 * series.  The renderer expects a dataset that:
 * <ul>
 * <li>has exactly two series;</li>
 * <li>each series has the same x-values;</li>
 * <li>no <code>null</code> values;
 * </ul>
 */
public class XYDifferenceRenderer extends AbstractXYItemRenderer 
                                  implements XYItemRenderer, 
                                             Cloneable,
                                             PublicCloneable,
                                             Serializable {

    /** The paint used to highlight positive differences (y(0) > y(1)). */
    private transient Paint positivePaint;

    /** The paint used to highlight negative differences (y(0) < y(1)). */
    private transient Paint negativePaint;

    /** Display shapes at each point? */
    private boolean plotShapes = true;

    /**
     * Creates a new renderer with default attributes.
     */
    public XYDifferenceRenderer() {
        this(Color.green, Color.red, false);
    }
    
    /**
     * Creates a new renderer.
     *
     * @param positivePaint  the highlight color for positive differences 
     *                       (<code>null</code> not permitted).
     * @param negativePaint  the highlight color for negative differences 
     *                       (<code>null</code> not permitted).
     * @param shapes  draw shapes?
     */
    public XYDifferenceRenderer(Paint positivePaint, Paint negativePaint, 
                                boolean shapes) {
        if (positivePaint == null) {
            throw new IllegalArgumentException(
                "Null 'positivePaint' argument."
            );
        }
        if (negativePaint == null) {
            throw new IllegalArgumentException(
                "Null 'negativePaint' argument."
            );
        }
        this.positivePaint = positivePaint;
        this.negativePaint = negativePaint;
        this.plotShapes = shapes;
    }

    /**
     * Returns the paint used to highlight positive differences.
     *
     * @return the paint (never <code>null</code>).
     */
    public Paint getPositivePaint() {
        return this.positivePaint;
    }

    /**
     * Sets the paint used to highlight positive differences.
     * 
     * @param paint  the paint (<code>null</code> not permitted).
     */
    public void setPositivePaint(Paint paint) {
        if (paint == null) {
            throw new IllegalArgumentException("Null 'paint' argument.");
        }
        this.positivePaint = paint;
        notifyListeners(new RendererChangeEvent(this));
    }

    /**
     * Returns the paint used to highlight negative differences.
     *
     * @return the paint (never <code>null</code>).
     */
    public Paint getNegativePaint() {
        return this.negativePaint;
    }
    
    /**
     * Sets the paint used to highlight negative differences.
     * 
     * @param paint  the paint (<code>null</code> not permitted).
     */
    public void setNegativePaint(Paint paint) {
        if (paint == null) {
            throw new IllegalArgumentException("Null 'paint' argument.");
        }
        this.negativePaint = paint;
        notifyListeners(new RendererChangeEvent(this));
    }

    /**
     * Returns a flag that controls whether or not shapes are drawn for each 
     * data value.
     * 
     * @return a boolean.
     */
    public boolean getPlotShapes() {
        return this.plotShapes;
    }

    /**
     * Sets a flag that controls whether or not shapes are drawn for each 
     * data value.
     * 
     * @param flag  the flag.
     */
    public void setPlotShapes(boolean flag) {
        this.plotShapes = flag;
        notifyListeners(new RendererChangeEvent(this));
    }
    
    /**
     * Initialises the renderer and returns a state object that should be 
     * passed to subsequent calls to the drawItem() method.  This method will 
     * be called before the first item is rendered, giving the renderer an 
     * opportunity to initialise any state information it wants to maintain.  
     * The renderer can do nothing if it chooses.
     *
     * @param g2  the graphics device.
     * @param dataArea  the area inside the axes.
     * @param plot  the plot.
     * @param data  the data.
     * @param info  an optional info collection object to return data back to 
     *              the caller.
     *
     * @return a state object.
     */
    public XYItemRendererState initialise(Graphics2D g2,
                                          Rectangle2D dataArea,
                                          XYPlot plot,
                                          XYDataset data,
                                          PlotRenderingInfo info) {

        return super.initialise(g2, dataArea, plot, data, info);

    }

    /**
     * Returns <code>2</code>, the number of passes required by the renderer.  
     * The {@link XYPlot} will run through the dataset this number of times.
     * 
     * @return the number of passes required by the renderer.
     */
    public int getPassCount() {
        return 2;
    }
    
    /**
     * Draws the visual representation of a single data item.
     *
     * @param g2  the graphics device.
     * @param state  the renderer state.
     * @param dataArea  the area within which the data is being drawn.
     * @param info  collects information about the drawing.
     * @param plot  the plot (can be used to obtain standard color 
     *              information etc).
     * @param domainAxis  the domain (horizontal) axis.
     * @param rangeAxis  the range (vertical) axis.
     * @param dataset  the dataset.
     * @param series  the series index (zero-based).
     * @param item  the item index (zero-based).
     * @param crosshairState  crosshair information for the plot 
     *                        (<code>null</code> permitted).
     * @param pass  the pass index.
     */
    public void drawItem(Graphics2D g2,
                         XYItemRendererState state,
                         Rectangle2D dataArea,
                         PlotRenderingInfo info,
                         XYPlot plot,
                         ValueAxis domainAxis,
                         ValueAxis rangeAxis,
                         XYDataset dataset,
                         int series,
                         int item,
                         CrosshairState crosshairState,
                         int pass) {

        if (pass == 0) {
            drawItemPass0(
                g2, dataArea, info, plot, domainAxis, rangeAxis, dataset,
                series, item, crosshairState
            );
        }
        else if (pass == 1) {
            drawItemPass1(
                g2, dataArea, info, plot, domainAxis, rangeAxis, dataset,
                series, item, crosshairState
            );
        }

    }

    /**
     * Draws the visual representation of a single data item, first pass.
     *
     * @param g2  the graphics device.
     * @param dataArea  the area within which the data is being drawn.
     * @param info  collects information about the drawing.
     * @param plot  the plot (can be used to obtain standard color 
     *              information etc).
     * @param domainAxis  the domain (horizontal) axis.
     * @param rangeAxis  the range (vertical) axis.
     * @param dataset  the dataset.
     * @param series  the series index (zero-based).
     * @param item  the item index (zero-based).
     * @param crosshairState  crosshair information for the plot 
     *                        (<code>null</code> permitted).
     */
    protected void drawItemPass0(Graphics2D g2,
                                 Rectangle2D dataArea,
                                 PlotRenderingInfo info,
                                 XYPlot plot,
                                 ValueAxis domainAxis,
                                 ValueAxis rangeAxis,
                                 XYDataset dataset,
                                 int series,
                                 int item,
                                 CrosshairState crosshairState) {

        if (series == 0) {

            PlotOrientation orientation = plot.getOrientation();
            RectangleEdge domainAxisLocation = plot.getDomainAxisEdge();
            RectangleEdge rangeAxisLocation = plot.getRangeAxisEdge();
            
            double y0 = dataset.getYValue(0, item);
            double x1 = dataset.getXValue(1, item);
            double y1 = dataset.getYValue(1, item);

            double transY0 = rangeAxis.valueToJava2D(
                y0, dataArea, rangeAxisLocation
            );
            double transX1 = domainAxis.valueToJava2D(
                x1, dataArea, domainAxisLocation
            );
            double transY1 = rangeAxis.valueToJava2D(
                y1, dataArea, rangeAxisLocation
            );

            if (item > 0) {
                double prevx0 = dataset.getXValue(0, item - 1);
                double prevy0 = dataset.getYValue(0, item - 1);
                double prevy1 = dataset.getYValue(1, item - 1);

                double prevtransX0 = domainAxis.valueToJava2D(
                    prevx0, dataArea, domainAxisLocation
                );
                double prevtransY0 = rangeAxis.valueToJava2D(
                    prevy0, dataArea, rangeAxisLocation
                );
                double prevtransY1 = rangeAxis.valueToJava2D(
                    prevy1, dataArea, rangeAxisLocation
                );

                Shape positive = getPositiveArea(
                    (float) prevtransX0, (float) prevtransY0, 
                    (float) prevtransY1,
                    (float) transX1, (float) transY0, (float) transY1,
                    orientation
                );
                if (positive != null) {
                    g2.setPaint(getPositivePaint());
                    g2.fill(positive);
                }

                Shape negative = getNegativeArea(
                    (float) prevtransX0, (float) prevtransY0, 
                    (float) prevtransY1,
                    (float) transX1, (float) transY0, (float) transY1,
                    orientation
                );

                if (negative != null) {
                    g2.setPaint(getNegativePaint());
                    g2.fill(negative);
                }
            }
        }

    }

    /**
     * Draws the visual representation of a single data item, second pass.  In 
     * the second pass, the renderer draws the lines and shapes for the 
     * individual points in the two series.
     *
     * @param g2  the graphics device.
     * @param dataArea  the area within which the data is being drawn.
     * @param info  collects information about the drawing.
     * @param plot  the plot (can be used to obtain standard color information 
     *              etc).
     * @param domainAxis  the domain (horizontal) axis.
     * @param rangeAxis  the range (vertical) axis.
     * @param dataset  the dataset.
     * @param series  the series index (zero-based).
     * @param item  the item index (zero-based).
     * @param crosshairState  crosshair information for the plot 
     *                        (<code>null</code> permitted).
     */
    protected void drawItemPass1(Graphics2D g2,
                                 Rectangle2D dataArea,
                                 PlotRenderingInfo info,
                                 XYPlot plot,
                                 ValueAxis domainAxis,
                                 ValueAxis rangeAxis,
                                 XYDataset dataset,
                                 int series,
                                 int item,
                                 CrosshairState crosshairState) {

        Shape entityArea = null;
        EntityCollection entities = null;
        if (info != null) {
            entities = info.getOwner().getEntityCollection();
        }

        Paint seriesPaint = getItemPaint(series, item);
        Stroke seriesStroke = getItemStroke(series, item);
        g2.setPaint(seriesPaint);
        g2.setStroke(seriesStroke);

        if (series == 0) {

            PlotOrientation orientation = plot.getOrientation(); 
            RectangleEdge domainAxisLocation = plot.getDomainAxisEdge();
            RectangleEdge rangeAxisLocation = plot.getRangeAxisEdge();

            double x0 = dataset.getXValue(0, item);
            double y0 = dataset.getYValue(0, item);
            double x1 = dataset.getXValue(1, item);
            double y1 = dataset.getYValue(1, item);

            double transX0 = domainAxis.valueToJava2D(
                x0, dataArea, domainAxisLocation
            );
            double transY0 = rangeAxis.valueToJava2D(
                y0, dataArea, rangeAxisLocation
            );
            double transX1 = domainAxis.valueToJava2D(
                x1, dataArea, domainAxisLocation
            );
            double transY1 = rangeAxis.valueToJava2D(
                y1, dataArea, rangeAxisLocation
            );

            if (item > 0) {
                // get the previous data points...
                double prevx0 = dataset.getXValue(0, item - 1);
                double prevy0 = dataset.getYValue(0, item - 1);
                double prevx1 = dataset.getXValue(1, item - 1);
                double prevy1 = dataset.getYValue(1, item - 1);

                double prevtransX0 = domainAxis.valueToJava2D(
                    prevx0, dataArea, domainAxisLocation
                );
                double prevtransY0 = rangeAxis.valueToJava2D(
                    prevy0, dataArea, rangeAxisLocation
                );
                double prevtransX1 = domainAxis.valueToJava2D(
                    prevx1, dataArea, domainAxisLocation
                );
                double prevtransY1 = rangeAxis.valueToJava2D(
                    prevy1, dataArea, rangeAxisLocation
                );

                Line2D line0 = null;
                Line2D line1 = null;
                if (orientation == PlotOrientation.HORIZONTAL) {
                    line0 = new Line2D.Double(
                        transY0, transX0, prevtransY0, prevtransX0
                    );
                    line1 = new Line2D.Double(
                        transY1, transX1, prevtransY1, prevtransX1
                    );
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    line0 = new Line2D.Double(
                        transX0, transY0, prevtransX0, prevtransY0
                    );
                    line1 = new Line2D.Double(
                        transX1, transY1, prevtransX1, prevtransY1
                    );
                }
                if (line0 != null && line0.intersects(dataArea)) {
                    g2.setPaint(getItemPaint(series, item));
                    g2.draw(line0);
                }
                if (line1 != null && line1.intersects(dataArea)) {
                    g2.setPaint(getItemPaint(1, item));
                    g2.draw(line1);
                }
            }

            if (getPlotShapes()) {
                Shape shape0 = getItemShape(series, item);
                if (orientation == PlotOrientation.HORIZONTAL) {
                    shape0 = ShapeUtilities.createTranslatedShape(
                        shape0, transY0, transX0
                    );
                }
                else {  // vertical
                    shape0 = ShapeUtilities.createTranslatedShape(
                        shape0, transX0, transY0
                    );
                }
                if (shape0.intersects(dataArea)) {
                    g2.setPaint(getItemPaint(series, item));
                    g2.fill(shape0);
                }
                entityArea = shape0;

                // add an entity for the item...
                if (entities != null) {
                    if (entityArea == null) {
                        entityArea = new Rectangle2D.Double(
                            transX0 - 2, transY0 - 2, 4, 4
                        );
                    }
                    String tip = null;
                    XYToolTipGenerator generator = getToolTipGenerator(
                        series, item
                    );
                    if (generator != null) {
                        tip = generator.generateToolTip(dataset, series, item);
                    }
                    String url = null;
                    if (getURLGenerator() != null) {
                        url = getURLGenerator().generateURL(
                            dataset, series, item
                        );
                    }
                    XYItemEntity entity = new XYItemEntity(
                        entityArea, dataset, series, item, tip, url
                    );
                    entities.add(entity);
                }

                Shape shape1 = getItemShape(series + 1, item);
                shape1 = ShapeUtilities.createTranslatedShape(
                    shape1, transX1, transY1
                );
                if (shape1.intersects(dataArea)) {
                    g2.setPaint(getItemPaint(series + 1, item));
                    g2.fill(shape1);
                }
                entityArea = shape1;

                // add an entity for the item...
                if (entities != null) {
                    if (entityArea == null) {
                        entityArea = new Rectangle2D.Double(
                            transX1 - 2, transY1 - 2, 4, 4
                        );
                    }
                    String tip = null;
                    XYToolTipGenerator generator = getToolTipGenerator(
                        series, item
                    );
                    if (generator != null) {
                        tip = generator.generateToolTip(
                            dataset, series + 1, item
                        );
                    }
                    String url = null;
                    if (getURLGenerator() != null) {
                        url = getURLGenerator().generateURL(
                            dataset, series + 1, item
                        );
                    }
                    XYItemEntity entity = new XYItemEntity(
                        entityArea, dataset, series + 1, item, tip, url
                    );
                    entities.add(entity);
                }
            }
            updateCrosshairValues(
                crosshairState, x1, y1, transX1, transY1, orientation
            );
        }

    }

    /**
     * Returns the positive area for a crossover point.
     * 
     * @param x0  x coordinate.
     * @param y0A  y coordinate A.
     * @param y0B  y coordinate B.
     * @param x1  x coordinate.
     * @param y1A  y coordinate A.
     * @param y1B  y coordinate B.
     * @param orientation  the plot orientation.
     * 
     * @return The positive area.
     */
    protected Shape getPositiveArea(float x0, float y0A, float y0B, 
                                    float x1, float y1A, float y1B,
                                    PlotOrientation orientation) {

        Shape result = null;

        boolean startsNegative = (y0A >= y0B);  
        boolean endsNegative = (y1A >= y1B);
        if (orientation == PlotOrientation.HORIZONTAL) {
            startsNegative = (y0B >= y0A);
            endsNegative = (y1B >= y1A);
        }
        
        if (startsNegative) {  // starts negative
            if (endsNegative) {
                // all negative - return null
                result = null;
            }
            else {
                // changed from negative to positive
                float[] p = getIntersection(x0, y0A, x1, y1A, x0, y0B, x1, y1B);
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y1A, x1);
                    area.lineTo(p[1], p[0]);
                    area.lineTo(y1B, x1);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x1, y1A);
                    area.lineTo(p[0], p[1]);
                    area.lineTo(x1, y1B);
                    area.closePath();
                }
                result = area;
            }
        }
        else {  // starts positive
            if (endsNegative) {
                // changed from positive to negative
                float[] p = getIntersection(x0, y0A, x1, y1A, x0, y0B, x1, y1B);
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y0A, x0);
                    area.lineTo(p[1], p[0]);
                    area.lineTo(y0B, x0);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x0, y0A);
                    area.lineTo(p[0], p[1]);
                    area.lineTo(x0, y0B);
                    area.closePath();
                }
                result = area;

            }
            else {
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y0A, x0);
                    area.lineTo(y1A, x1);
                    area.lineTo(y1B, x1);
                    area.lineTo(y0B, x0);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x0, y0A);
                    area.lineTo(x1, y1A);
                    area.lineTo(x1, y1B);
                    area.lineTo(x0, y0B);
                    area.closePath();
                }
                result = area;
            }

        }

        return result;

    }

    /**
     * Returns the negative area for a cross-over section.
     * 
     * @param x0  x coordinate.
     * @param y0A  y coordinate A.
     * @param y0B  y coordinate B.
     * @param x1  x coordinate.
     * @param y1A  y coordinate A.
     * @param y1B  y coordinate B.
     * @param orientation  the plot orientation.
     * 
     * @return The negative area.
     */
    protected Shape getNegativeArea(float x0, float y0A, float y0B, 
                                    float x1, float y1A, float y1B,
                                    PlotOrientation orientation) {

        Shape result = null;

        boolean startsNegative = (y0A >= y0B);
        boolean endsNegative = (y1A >= y1B);
        if (orientation == PlotOrientation.HORIZONTAL) {
            startsNegative = (y0B >= y0A);
            endsNegative = (y1B >= y1A);
        }
        if (startsNegative) {  // starts negative
            if (endsNegative) {  // all negative
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y0A, x0);
                    area.lineTo(y1A, x1);
                    area.lineTo(y1B, x1);
                    area.lineTo(y0B, x0);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x0, y0A);
                    area.lineTo(x1, y1A);
                    area.lineTo(x1, y1B);
                    area.lineTo(x0, y0B);
                    area.closePath();
                }
                result = area;
            }
            else {  // changed from negative to positive
                float[] p = getIntersection(x0, y0A, x1, y1A, x0, y0B, x1, y1B);
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y0A, x0);
                    area.lineTo(p[1], p[0]);
                    area.lineTo(y0B, x0);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x0, y0A);
                    area.lineTo(p[0], p[1]);
                    area.lineTo(x0, y0B);
                    area.closePath();
                }
                result = area;
            }
        }
        else {
            if (endsNegative) {
                // changed from positive to negative
                float[] p = getIntersection(x0, y0A, x1, y1A, x0, y0B, x1, y1B);
                GeneralPath area = new GeneralPath();
                if (orientation == PlotOrientation.HORIZONTAL) {
                    area.moveTo(y1A, x1);
                    area.lineTo(p[1], p[0]);
                    area.lineTo(y1B, x1);
                    area.closePath();
                }
                else if (orientation == PlotOrientation.VERTICAL) {
                    area.moveTo(x1, y1A);
                    area.lineTo(p[0], p[1]);
                    area.lineTo(x1, y1B);
                    area.closePath();
                }
                result = area;
            }
            else {
                // all negative - return null
            }

        }

        return result;

    }

    /**
     * Returns the intersection point of two lines.
     * 
     * @param x1  x1
     * @param y1  y1
     * @param x2  x2
     * @param y2  y2
     * @param x3  x3
     * @param y3  y3
     * @param x4  x4
     * @param y4  y4
     * 
     * @return The intersection point.
     */
    private float[] getIntersection(float x1, float y1, float x2, float y2,
                                    float x3, float y3, float x4, float y4) {

        float n = (x4 - x3) * (y1 - y3) - (y4 - y3) * (x1 - x3);
        float d = (y4 - y3) * (x2 - x1) - (x4 - x3) * (y2 - y1);
        float u = n / d;

        float[] result = new float[2];
        result[0] = x1 + u * (x2 - x1);
        result[1] = y1 + u * (y2 - y1);
        return result;

    }
    
    /**
     * Returns a default legend item for the specified series.  Subclasses 
     * should override this method to generate customised items.
     *
     * @param datasetIndex  the dataset index (zero-based).
     * @param series  the series index (zero-based).
     *
     * @return A legend item for the series.
     */
    public LegendItem getLegendItem(int datasetIndex, int series) {
        LegendItem result = null;
        XYPlot p = getPlot();
        if (p != null) {
            XYDataset dataset = p.getDataset(datasetIndex);
            if (dataset != null) {
                if (getItemVisible(series, 0)) {
                    String label = getLegendItemLabelGenerator().generateLabel(
                        dataset, series
                    );
                    String description = label;
                    Paint paint = getSeriesPaint(series);
                    Stroke stroke = getSeriesStroke(series);
                    // TODO:  the following hard-coded line needs generalising
                    Line2D line = new Line2D.Double(-7.0, 0.0, 7.0, 0.0);
                    result = new LegendItem(
                        label, description, line, stroke, paint
                    );
                }
            }

        }

        return result;

    }

    /**
     * Returns a clone of the renderer.
     * 
     * @return A clone.
     * 
     * @throws CloneNotSupportedException  if the renderer cannot be cloned.
     */
    public Object clone() throws CloneNotSupportedException {
        return super.clone();
    }

    /**
     * Provides serialization support.
     *
     * @param stream  the output stream.
     *
     * @throws IOException  if there is an I/O error.
     */
    private void writeObject(ObjectOutputStream stream) throws IOException {
        stream.defaultWriteObject();
        SerialUtilities.writePaint(this.positivePaint, stream);
        SerialUtilities.writePaint(this.negativePaint, stream);
    }

    /**
     * Provides serialization support.
     *
     * @param stream  the input stream.
     *
     * @throws IOException  if there is an I/O error.
     * @throws ClassNotFoundException  if there is a classpath problem.
     */
    private void readObject(ObjectInputStream stream) 
        throws IOException, ClassNotFoundException {
        stream.defaultReadObject();
        this.positivePaint = SerialUtilities.readPaint(stream);
        this.negativePaint = SerialUtilities.readPaint(stream);
    }

}