title: matplotlib align twinx tick marks
body: <p>Is it possible to make a plot with two independent y-axes such that the tick marks align?</p>
  
  <p>Below is an example of half of the solution. I've doubled the y-axis using <code>twinx</code>, but the tick marks aren't aligned and the gridlines form an awkward pattern on the plot. Is there a way to make the tick marks share the same positions, but correspond to different y-values? In the example below, I would like the tick mark for 5 on the left to be at the same vertical position as the tick mark for 6 on the right.</p>
  
  <pre><code>import numpy as np
  
  a = np.random.normal(10, 3, size=20)
  b = np.random.normal(20, 5, size=40)
  
  fig, ax1 = plt.subplots()
  ax2 = ax1.twinx()
  ax1.hist(a)
  ax2.hist(b)
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/F0kur.png" alt="histograms"></p>
  
  <p>The overarching point of this exercise is to make the grid lines overlap for the two axes.</p>