public class graphWriter extends JPanel {
    private int
      markerX = 0,
      markerY = 0,
      width = 0,
      height = 0;
    
    Image bufImage = null;
    
    public graphWriter() {
        addMouseMotionListener(
          new MouseMotionListener() {
            public void mouseDragged(MouseEvent e) {
            }
            
            public void mouseMoved(MouseEvent e) {
                //Get coordinates of the mouse
                markerX = e.getX();
                markerY = e.getY();
                repaint();
            }
        }
        );        
    }
    
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        if (bufImage==null){
            bufImage = createImage(getWidth(), getHeight());
            Graphics gBuf = bufImage.getGraphics();
            //Draw line
            gBuf.drawLine(0, 0, getWidth(), getHeight());
        }

        //Get width and height of current clip
        width  = (int)g.getClipBounds().getWidth();
        height = (int)(g.getClipBounds().getHeight());
        
        //Draw line
        g.drawImage(bufImage,0,0,this);
        
        //Draw two lines, one horizontal, and one vertical to create the cursor
        g.drawLine(markerX, 0, markerX, height);
        g.drawLine(0, markerY, width, markerY);
    }
}