//draws a line on user's screen between most recent mouse position, and second most recent
//then updates second most recent position
public void DrawLine()
{
if (oldX != -1 && oldY != -1)
{
Graphics g = SignaturePanel.CreateGraphics();
Pen pen = new Pen(Color.Black);
g.DrawLine(pen, newX, newY, oldX, oldY);
}
oldX = newX;
oldY = newY;
}
