[hardwired]

The next thing to do is check on the data that shows up in GraphPanel with something like this:

Code:

public GraphPanel(float[] p, float[] i)
{
    principleData = p;
    interestData = i;
    System.out.printf("principalData = %s%ninterestData = %s%n",
                       java.util.Arrays.toString(principalData),
                       java.util.Arrays.toString(interestData));
    font = new Font("lucida sans regular", Font.PLAIN, 16);
    setBackground(Color.white);
}

Start tracking down where things aren't going the way you'd like.