title: Add annotation in an interactive plot
body: <p>I m trying to add an annotation at the middle of my interactive plot
  I would like to see my <em>i</em> value of the loop generating my <em>test</em> list
  with all my data. For each imshow plot, i would like to see my <em>i</em> value,
  i add an ax.annotate but it doesnt work.</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  import matplotlib.animation as animation
  fig = plt.figure() # make figure
  ax = fig.add_subplot(111)
  
  test = []
  mask2 = np.random.randint(255, size=(20, 20))
  
  for i in range(1,5,3):
    kernel = cv2.getStructuringElement(cv2.MORPH_ELLIPSE,(i,i))
    res = (cv2.morphologyEx(mask2.astype(uint8),cv2.MORPH_OPEN,kernel))
    #plt.imshow(res,cmap=plt.cm.gray,alpha=1);plt.show()
    test.append(res)
  
  
  # make axesimage object
  # the vmin and vmax here are very important to get the color map correct
  im = ax.imshow(test[0], cmap=plt.get_cmap('hot'), vmin=0, vmax=255)
  im2 = ax.annotate('This is awesome!', 
               xy=(76, -10.75),  
               xycoords='data',
               textcoords='offset points',
               arrowprops=dict(arrowstyle="-&gt;"))
  plt.show()
  
  # function to update figure
  def updatefig(j):
      # set the data in the axesimage object
      im.set_array(test[j])
      # return the artists set
      return im,
  # kick off the animation
  ani = animation.FuncAnimation(fig, updatefig, frames=range(20), 
                                interval=50, blit=True)
  plt.show()
  </code></pre>