title: Plot don't refresh when slider updated matplotlib
body: <p>I can't figure out why the plot don't refresh when the slider is updated.
  I'm using Jupiter notebook and I choose the backend with 'nbAgg' parameter.</p>
  <p>Initialization code :</p>
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  import matplotlib.animation as animation
  from matplotlib import use as m_use
  from matplotlib.widgets import Slider
  
  m_use('nbAgg')
  x1 = np.random.normal(-2.5, 1, 10000)
  x2 = np.random.gamma(2, 1.5, 10000)
  x3 = np.random.exponential(2, 10000)+7
  x4 = np.random.uniform(14,20, 10000)
  fig, ((ax1, ax2), (ax3, ax4)) = plt.subplots(2, 2,figsize=(7, 5))
  plt.subplots_adjust(left=0.1,right=.8)
  </code></pre>
  <p>There is a animation which lunch this function :</p>
  <pre><code>def updateData(curr,divider=7):
      global ax1, ax2, ax3, ax4
      for ax in (ax1, ax2, ax3, ax4):
          ax.cla()
      if curr &lt;= 679 :
          my_b = int(np.around(curr/divider)+3)
      else : my_b = 100
      
      multi = 100
      ax1.hist(x1[:curr*multi],bins=my_b)
      ax2.hist(x2[:curr*multi],bins=my_b)
      ax3.hist(x3[:curr*multi],bins=my_b)
      ax4.hist(x4[:curr*multi],bins=my_b)
      fig.suptitle('Frame {} on 100'.format((curr+1)))
      return None
  </code></pre>
  <p>The animation :</p>
  <pre><code>simulation = animation.FuncAnimation(fig=fig, func=updateData, frames=10,
                                       blit=False, interval=1, repeat=False)
  </code></pre>
  <p>Here the slider which stuck me :</p>
  <pre><code>slider_ax = plt.axes([.9, 0.45, 0.01, 0.3])
  slider = Slider(ax=slider_ax,label='Divider \nfor bins',valmin=1,valmax=15, valinit=7, orientation='vertical',valfmt='%.0f',track_color='black',facecolor='red',
                    handle_style={'facecolor':'k','edgecolor':'#86a2cf','size':20})
  def update():
      
      anim_2 = animation.FuncAnimation(fig=fig, func=updateData, frames=20,
                                       blit=False, interval=1, repeat=False)    
  </code></pre>
  <p>This below function don't work as expected :</p>
  <pre><code>slider.on_changed(update)
  
  simulation = animation.FuncAnimation(fig=fig, func=updateData, frames=10,
                                       blit=False, interval=1, repeat=False)
  plt.show()
  </code></pre>