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JFreeChart is a class library, written in Java, for generating charts. Utilising the Java2D API, it supports a wide range of chart types including bar charts, pie charts, line charts, XY-plots, time series plots, Sankey charts and more.
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/* ===========================================================
 * JFreeChart : a free chart library for the Java(tm) platform
 * ===========================================================
 *
 * (C) Copyright 2000-present, by David Gilbert 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., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301,
 * USA.
 *
 * [Oracle and Java are registered trademarks of Oracle and/or its affiliates. 
 * Other names may be trademarks of their respective owners.]
 *
 * -----------------
 * HeatMapUtils.java
 * -----------------
 * (C) Copyright 2009-present, by David Gilbert.
 *
 * Original Author:  David Gilbert;
 * Contributor(s):   -;
 *
 */

package org.jfree.data.general;

import java.awt.Graphics2D;
import java.awt.Paint;
import java.awt.image.BufferedImage;
import org.jfree.chart.renderer.PaintScale;
import org.jfree.chart.util.Args;
import org.jfree.data.xy.XYDataset;
import org.jfree.data.xy.XYSeries;
import org.jfree.data.xy.XYSeriesCollection;

/**
 * A utility class for the {@link HeatMapDataset}.
 */
public abstract class HeatMapUtils {

    /**
     * Returns a dataset containing one series that holds a copy of the (x, z)
     * data from one row (y-index) of the specified dataset.
     *
     * @param dataset  the dataset ({@code null} not permitted).
     * @param row  the row (y) index.
     * @param seriesName  the series name/key ({@code null} not permitted).
     *
     * @return The dataset.
     */
    public static XYDataset extractRowFromHeatMapDataset(HeatMapDataset dataset,
            int row, Comparable seriesName) {
        XYSeries series = new XYSeries(seriesName);
        int cols = dataset.getXSampleCount();
        for (int c = 0; c < cols; c++) {
            series.add(dataset.getXValue(c), dataset.getZValue(c, row));
        }
        XYSeriesCollection result = new XYSeriesCollection(series);
        return result;
    }

    /**
     * Returns a dataset containing one series that holds a copy of the (y, z)
     * data from one column (x-index) of the specified dataset.
     *
     * @param dataset  the dataset ({@code null} not permitted).
     * @param column  the column (x) index.
     * @param seriesName  the series name ({@code null} not permitted).
     *
     * @return The dataset.
     */
    public static XYDataset extractColumnFromHeatMapDataset(
            HeatMapDataset dataset, int column, Comparable seriesName) {
        XYSeries series = new XYSeries(seriesName);
        int rows = dataset.getYSampleCount();
        for (int r = 0; r < rows; r++) {
            series.add(dataset.getYValue(r), dataset.getZValue(column, r));
        }
        XYSeriesCollection result = new XYSeriesCollection(series);
        return result;
    }

    /**
     * Creates an image that displays the values from the specified dataset.
     *
     * @param dataset  the dataset ({@code null} not permitted).
     * @param paintScale  the paint scale for the z-values ({@code null}
     *         not permitted).
     *
     * @return A buffered image.
     */
    public static BufferedImage createHeatMapImage(HeatMapDataset dataset,
            PaintScale paintScale) {

        Args.nullNotPermitted(dataset, "dataset");
        Args.nullNotPermitted(paintScale, "paintScale");
        int xCount = dataset.getXSampleCount();
        int yCount = dataset.getYSampleCount();
        BufferedImage image = new BufferedImage(xCount, yCount,
                BufferedImage.TYPE_INT_ARGB);
        Graphics2D g2 = image.createGraphics();
        for (int xIndex = 0; xIndex < xCount; xIndex++) {
            for (int yIndex = 0; yIndex < yCount; yIndex++) {
                double z = dataset.getZValue(xIndex, yIndex);
                Paint p = paintScale.getPaint(z);
                g2.setPaint(p);
                g2.fillRect(xIndex, yCount - yIndex - 1, 1, 1);
            }
        }
        return image;
    }

}