title: Matplotlib, two x-axes labels on multipanel plot
body: <p>I would like to make a 3 panel plot (three panels vertically stacked on top of each other), but with two x-axis labels, e.g. from
  <a href="http://www.astropy.org/astropy-tutorials/edshift_plot.html" rel="nofollow noreferrer">here</a></p>
  
  <p>The lower x-axis and top x-axis are applicable to the data in all three panels.</p>
  
  <pre><code>fig, (ax1, ax2, ax3, ax4) = plt.subplots(4, sharex=True,
                             gridspec_kw={'height_ratios': [7,0, 7, 3]},
                             figsize=(14.0, 16.0))
  </code></pre>
  
  <p>with the top panel being </p>
  
  <pre><code>ax1. and ax2. 
  </code></pre>
  
  <p>and </p>
  
  <pre><code>ax2 = ax1.twiny()
  </code></pre>
  
  <p>doesn't seem to work.  </p>