title: matplotlib interactive subplots with sharex and twiny
body: <p>I am plotting a figure with <code>matplotlib</code> using the following code. I have two stacked subplots, as well as a added second x axis using <code>twiny</code>.</p>
  
  <pre><code>#!/usr/bin/python
  import os
  import numpy as np
  import matplotlib.pylab as mp
  
  # random data
  data = np.random.random((10,3)) 
  data[:,0] = np.linspace(0,1,10)
  
  # init figure
  fig, axs = mp.subplots(2, sharex=True)
  axs = np.append(axs,axs[1].twiny())
  
  # plot top
  axs[0].plot(data[:,0],data[:,1],'bo-',linewidth=2.0)
  axs[0].axis([data[0,0],data[-1,0],data[:,1].min(),data[:,1].max()])
  
  # plot bottom
  axs[1].plot(data[:,0],data[:,2],'rx-',linewidth=2.0)
  
  # add second axis
  axs[2].xaxis.set_ticks_position('bottom')
  axs[2].xaxis.set_label_position('bottom')
  axs[2].spines['bottom'].set_position(('outward', 40))
  axs[2].set_xlim(-180,180)
  
  # plot
  mp.tight_layout(pad=0.6)
  mp.show()
  </code></pre>
  
  <p>My problem is in the interactive plot window: if I pan the lower plot, all 3 x-axes move accordingly. If I pan the top plot, the added <code>twiny</code> axis does not move. Would this be possible in the current set up?</p>