title: xaml shapes - relative to canvas size
body: <p>I have the need to draw shapes in XAML that are always relative to the container window/canvas size.  For example, I need a window to show a 3x3 grid on it, where the grid always splits the window into 3 equal parts.  Can I create lines that are bound to 1/3 the width of the grid?</p>
  
  <p>thank you</p>