public void ZoomBy(int delta, Point relativeTo)
{
double ticks = delta / 120.0;
Point adjust = relativeTo;
double scaleStart = PageNavigation.Scale;
PageNavigation.ZoomBy(ticks / 3.0);
double scaleEnd = PageNavigation.Scale;
double scalar = 1 / scaleEnd - 1 / scaleStart;
PageNavigation.PanBy(new Point(adjust.X * scalar, adjust.Y * scalar));
}