title: Matplotlib: no effect of set_data in imshow for the plot
body: <p>I have a strange error which I can't fix without your help. After I set an image with <code>imshow</code> in matplotlib it stays the same all the time even if I change it with the method <code>set_data</code>. Just take a look on this example:</p>
  
  <pre><code>import numpy as np
  from matplotlib import pyplot as plt
  
  
  def newevent(event):
      haha[1,1] += 1
      img.set_data(haha)
      print img.get_array()    # the data is change at this point
      plt.draw()
  
  haha = np.zeros((2,2))
  img = plt.imshow(haha)
  print img.get_array()        # [[0,0],[0,0]]
  plt.connect('button_press_event', newevent)
  plt.show()
  </code></pre>
  
  <p>After I plot it, the method <code>set_data</code> doesn't change anything inside the plot. Can someone explain me why?</p>
  
  <p><strong>EDIT</strong></p>
  
  <p>Just added a few lines to point out what I actually want to do.
  I want to redraw the data after I press a mouse button. I don't want to delete the whole figure, because it would be stupid if only one thing changes. </p>