CGPoint ProcessTouch (CGPoint touchPoint)
{
nfloat x = NMath.Max (0, touchPoint.X);
x = NMath.Min (x, graphLayer.Frame.Width + leftMargin);
nfloat y = NMath.Max (0, touchPoint.Y);
y = NMath.Min (y, graphLayer.Frame.Height + bottomMargin);
return new CGPoint (x, y);
}