title: One secondary label missing when plotting x and y secondary axis
body: <p>I'm coming from this question <a href="https://stackoverflow.com/questions/22228889/matplotlib-two-x-axis-and-two-y-axis">Matplotlib: two x axis and two y axis</a> where I learned how to plot two <code>x</code> and <code>y</code> axis on the same plot.</p>
  
  <p>Here's a <code>MWE</code>:</p>
  
  <pre><code>import matplotlib.pyplot as plt
  import numpy as np
  
  # Generate random data.    
  x1 = np.random.randn(50)
  y1 = np.linspace(0, 1, 50)
  x2 = np.random.randn(20)+15.
  y2 = np.linspace(10, 20, 20)
  
  # Plot both curves.
  fig = plt.figure()
  
  ax1 = fig.add_subplot(111)
  ax1.set_xlabel('x_1')
  ax1.set_ylabel('y_1')
  plt.plot(x1, y1, c='r')
  
  ax2 = ax1.twinx().twiny()
  ax2.set_xlabel('x_2')
  ax2.set_ylabel('y_2')
  plt.ylim(min(y2), max(y2))
  ax2.plot(x2, y2, c='b')
  
  plt.show()
  </code></pre>
  
  <p>and this is the output:</p>
  
  <p><img src="https://i.stack.imgur.com/AoHuJ.png" alt="enter image description here"></p>
  
  <p>The right <code>y</code> axis and the top <code>x</code> axis correspond to the blue curve.</p>
  
  <p>As you can see, the second <code>y</code> label is missing even though it is defined. I've tried a number of different approaches but I can't get it to show. Am I doing something wrong?</p>
  
  <hr>
  
  <p>Add:</p>
  
  <p>apparently there's an issue with the line:</p>
  
  <pre><code>ax2 = ax1.twinx().twiny()
  </code></pre>
  
  <p>if I invert it like so:</p>
  
  <pre><code>ax2 = ax1.twiny().twinx()
  </code></pre>
  
  <p>then it's the second <code>x</code> label that will not show.</p>