When we want to keep the initial background color of a panel for a
plotting application, we do simply the following for java.awt.Panel:
<code>
public class PlotPanel extends Panel{
 int oldX, oldY, newX, newY;

 public PlotPanel(){ ...
   setBackground(Color.white);
   ...
 }

 public void update(Graphics g){ //suppress bg redrawing
   g.setColor(Color.blue); //pen color
   paint(g);
 }

 public void paint(Graphis g){
   g.drawLine(oldX, oldY, newX, newY); //one stroke of plotting
 }
 .....
}
</code>
But, when we use a JPanel instead of Panel for a plotting canvas,
suddenly things become complicated and I have not found the solution
yet after struggling almost a full day. Let me repeat the problem:
how could we keep the initial background color for plotting on a
JPanel? ...I don't override update() method for JPanel version
of the PlotPanel class during the struggle because the spec of
the method is different for JComponent from awt.Component.