title: Matplotlib FuncAnimation, error when blit = true.
body: <p>I am trying to make an animation of an expanding circle using FuncAnimation and circle.set_radius(). However, the animation only works when blit = False. The code is as follow: </p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  from matplotlib import animation
  
  fig, ax = plt.subplots()
  plt.grid(True)
  plt.axis([-0.6, 0.6, -0.6, 0.6])
  circle1= plt.Circle([0,0],0.01,color="0.",fill=False, clip_on = True)
  ax.add_patch(circle1)
  
  dt = 1.0/20
  vel = 0.1
  
  def init():
      circle1.set_radius(0.01)
  
  def animate(i):
      global dt, vel
      r = vel * i * dt
      circle1.set_radius(r)
      return circle1,
  
  anim = animation.FuncAnimation(fig, animate, init_func=init,
                                 frames=200, interval= 50, blit=True)
  </code></pre>
  
  <p>It returns this error: </p>
  
  <pre><code>Traceback (most recent call last):
    File "/usr/local/lib/python2.7/site-packages/matplotlib/artist.py", line 61, in draw_wrapper
      draw(artist, renderer, *args, **kwargs)
    File "/usr/local/lib/python2.7/site-packages/matplotlib/figure.py", line 1139, in draw
      self.canvas.draw_event(renderer)
    File "/usr/local/lib/python2.7/site-packages/matplotlib/backend_bases.py", line 1809, in draw_event
      self.callbacks.process(s, event)
    File "/usr/local/lib/python2.7/site-packages/matplotlib/cbook.py", line 562, in process
      proxy(*args, **kwargs)
    File "/usr/local/lib/python2.7/site-packages/matplotlib/cbook.py", line 429, in __call__
      return mtd(*args, **kwargs)
    File "/usr/local/lib/python2.7/site-packages/matplotlib/animation.py", line 620, in _start
      self._init_draw()
    File "/usr/local/lib/python2.7/site-packages/matplotlib/animation.py", line 1166, in _init_draw
      for a in self._drawn_artists:
  TypeError: 'NoneType' object is not utterable
  </code></pre>
  
  <p>I am using mac OS. The animation will work when I changed blit = False. However, whenever I move my mouse there animation slow down. This is problematic because I have a separate thread generating the sound output. In practice, the circle will hit some data points and make sound. As a result they are not in sync. Please help. </p>