title: canvas.restore_region() not working
body: <p>I was trying to follow the instructions on:
  <a href="http://wiki.scipy.org/Cookbook/Matplotlib/Animations" rel="nofollow">http://wiki.scipy.org/Cookbook/Matplotlib/Animations</a> </p>
  
  <p>Why is the code below not showing the expected behaviour? (see comments in code on what is expected vs. what is observed)</p>
  
  <pre><code>import matplotlib.pyplot as plt
  from pylab import rand
  
  f, ax = plt.subplots()
  
  # Generate dummy plot
  ax.set_xlim([0, 10])
  ax.set_ylim([0, 10])
  ax.imshow(rand(10,10),  interpolation='nearest')
  # Until here it works
  # If i would add a 
  #     f.show() 
  # i would see the image
  
  # Save figure
  canvas = ax.figure.canvas
  background = canvas.copy_from_bbox(ax.bbox)  
  
  # Clear Axes
  ax.cla()
  
  # try to restore figure
  canvas.restore_region(background)
  f.show()
  
  # Unfortunatly I only see an empty figure now, Why??
  </code></pre>
  
  <p>Follow-up question:</p>
  
  <ul>
  <li>I intent to generate a semi transparent copy of <code>background</code> on an modified Position in the same axes. (How) Is this possible?</li>
  </ul>