title: pyplot twinx() tickmark overlap
body: <p>Pardon my poor python skills, I am rather new to the language!</p>
  <p>None the less, I am confused by the results I am getting from <code>twinx()</code> currently. I am not sure why when I twin the x axis, the ticks on the right hand side y axis seem to double.</p>
  <pre><code>import matplotlib.pyplot as plt
  
  x = linspace(0,2*pi,100)
  y = sin(x) + 100*rand(len(x))
  z = cos(x) + 100*rand(len(x))
  data = []
  data.append(y)
  data.append(z)
  
  fig = plt.figure(1)
  for kk in range(len(data)):
      ax1 = fig.add_subplot(111)
      ax1.plot(x.T, data[kk], 'b.-')
  
  plt.show()
  </code></pre>
  <p>The first plot displays (to my mind) the correct behavior
  <img src="https://i.stack.imgur.com/ucLdN.png" alt="left Y axis" /></p>
  <pre><code>fig2 = plt.figure(2)
  for kk in range(len(data)):
      ax3 = fig2.add_subplot(111)
      ax4 = ax3.twinx()
      ax4.plot(x.T, data[kk], 'b.-')
  
  plt.show()
  </code></pre>
  <p>While the second plot (where all I have done is flip the axis) seems to have poor y tick behavior wherein the two 'curves' each get their own tick marks.
  <img src="https://i.stack.imgur.com/TMjQ6.png" alt="right y axis" /></p>
  <p>Any thoughts as to why this might be occurring would be greatly appreciated!</p>