title: Setting second x-axis using set_xticklabels based on other subplot of same figure gives Text(X, u'X') representation
body: <p>I'm trying to set a second x-axis using xticklabels from another subplot of the same figure:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import matplotlib.gridspec as gridspec
  
  
  gs = gridspec.GridSpec(1,2)
  ax1 = plt.subplot(gs[0])
  ax2 = plt.subplot(gs[1])
  ax1.plot([1,2,3,4,5],[1,2,3,4,5])
  ax2.plot([4,5,6,7,8],[1,2,3,4,5])
  ax3 = ax2.twiny()
  ax3.set_xticklabels(ax1.get_xticklabels())
  plt.show()
  </code></pre>
  
  <p>The resulting plot however gives me a textual representation of the matplotlib.text object:</p>
  
  <p><a href="https://i.stack.imgur.com/0AAnY.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0AAnY.jpg" alt="xticklabel representation"></a></p>
  
  <p>any ideas what's going wrong here?
  Thanks!</p>