title: matplotlib add rectangle to Figure not to Axes
body: <p>I need to add a semi transparent skin over my matplotlib figure. I was thinking about adding a rectangle to the figure with alpha &lt;1  and a zorder high enough so its drawn on top of everything.</p>
  
  <p>I was thinking about something like that</p>
  
  <pre><code>figure.add_patch(Rectangle((0,0),1,1, alpha=0.5, zorder=1000))
  </code></pre>
  
  <p>But I guess rectangles are handled by Axes only. is there any turn around ?</p>