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Implementing BitmapChart
 
Entrance has an extensible, Java-based chart system,  and it was easy to write a first BitmapChart implementation  (with a hardwired bitmap) by extending ScatterChart.   Here is the code:
 
/*
 *  Copyright (c) 2008 Tod Landis All Rights Reserved
 *  Released subject to the GPL, v2
 */
 
package com.dbentrance.charts;
 
import java.awt.Graphics2D;
import java.awt.geom.AffineTransform;
import java.awt.image.BufferedImage;
import java.io.File;
import javax.imageio.ImageIO;
 
/**
 * A BitmapChart is an XY chart drawn on a bitmap.
 */
public class BitmapChart extends ScatterChart {
    // this will be "bitmap"
    private String fileName = "/Users/todlandis/proj/maps/PathfinderMap1024.jpg";
    
    @Override
    public void draw(Graphics2D g) {
        File f = new File(fileName);
        try {
            BufferedImage bi = ImageIO.read(f);
            g.drawImage(bi, new AffineTransform(), null);
            
            setClearBackground(false);
            getXAxis().setDrawAxis(false);
            getYAxis().setDrawAxis(false);
            
            super.draw(g);
        } catch(Exception e) {
            System.err.println(e);
        }
    }
    
    @Override
    public void calculateFrame(Graphics2D g) {
        setFrame(0, 0, 1024, 512);
        setFrameToFit(false);
    }
}
 
With small changes, eg. to allow the user to set the bitmap, this became BitmapChart in Entrance. 

BitmapChart is in turn extended by EarthChart, which draws an image of the Earth and makes it easy to plot lat,lon points onto it.   You'll find source for ScatterChart, BitmapChart, and EarthChart in the Entrance community download.