title: Share axes in matplotlib for only part of the subplots
body: <p>I am having a big plot where I initiated with:</p>
  
  <pre><code>import numpy as np
  import matplotlib.pyplot as plt
  
  fig, axs = plt.subplots(5, 4)
  </code></pre>
  
  <p>And I want to do share-x-axis between column 1 and 2; and do the same between column 3 and 4. However, column 1 and 2 does not share the same axis with column 3 and 4.</p>
  
  <p>I was wondering that would there be anyway to do this, and not <code>sharex=True</code> and <code>sharey=True</code> across all figures?</p>
  
  <p>PS: <a href="http://matplotlib.org/examples/pylab_examples/subplots_demo.html" rel="noreferrer">This tutorial</a> does not help too much, because it is only about sharing x/y within each row/column; they cannot do axis sharing between different rows/columns (unless share them across all axes).</p>