Code

private void ZoomIn(object sender, RoutedEventArgs e)
{
	Zoom(20);
}

private void ZoomOut(object sender, RoutedEventArgs e)
{
Zoom(-20);
}

private void Zoom(double zoom)
{
	Point oldcurpos = diagram.ClientToDoc(new Point(scrollViewer.ViewportWidth / 2, scrollViewer.ViewportHeight / 2));

	diagram.ZoomFactor += zoom;

	Point newcurpos = diagram.ClientToDoc(new Point(scrollViewer.ViewportWidth / 2, scrollViewer.ViewportHeight / 2));
	double diffx = (oldcurpos.X - newcurpos.X);
	double diffy = (oldcurpos.Y - newcurpos.Y);

	diagram.ScrollTo(new Point(
	diagram.ScrollX + diffx < 0 ? 0 : diagram.ScrollX + diffx,
	diagram.ScrollY + diffy < 0 ? 0 : diagram.ScrollY + diffy));
}