/// <summary>
/// performs a relative transformation during drag operations
/// </summary>
/// <param name="target"></param>
/// <param name="xy"></param>
/// <param name="g"></param>
protected void TransformDrag(FrameworkElement target, Point xy, TransformGroup g)
{
target.RelativeTransformPositionTo(new Point(xy.X - ClickPoint.X, xy.Y - ClickPoint.Y));
}
