title: How to stack a canvas and a button?
body: <p>I want to write the following application. There is a Canvas
  and a Button stacked vertically in a LinearView. When the button
  is pressed the first time a circle is drawn in the canvas, then
  if pressed again the circle disappears. The circle must appear
  centered in its space.</p>
  
  <p>Any ideas?</p>
  
  <p>Thanks,</p>
  
  <p>JG</p>