title: zorder in matplotlib with shared x_axis doesn't put on top
body: <p>This is my code, and the output figure:</p>
  
  <pre><code>fig, ax1 = plt.subplots()
  
  
  color = 'tab:red'
  ax1.set_xlabel('Iterations')
  ax1.set_ylabel('Non metric loss', color=color)
  ax1.plot(net_loss_bigger, color=color, zorder=3)
  ax1.tick_params(axis='y', labelcolor=color)
  
  ax2 = ax1.twinx()  # instantiate a second axes that shares the same x-axis
  
  color = 'tab:blue'
  ax2.set_ylabel('metric supervised loss', color=color)  # we already handled the x-label with ax1
  ax2.plot(metric_super_loss_bigger, color=color, zorder=2)
  ax2.tick_params(axis='y', labelcolor=color)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/gSUdr.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/gSUdr.png" alt="enter image description here"></a>
  <br>
  Ideas what I'm doing wrong? <br>
  Without the shared <code>ax1.twinx()</code> it works as expected. workarounds?<br></p>