title: twiny() in matplotlib changes the y-axis scale
body: <p>Plotting some data with matplotlib, it happens that inserting the <code>ax.twiny()</code> module changes the scale of the y axis.
  Before I use it:
  <img src="https://i.stack.imgur.com/qMfmp.png" alt="enter image description here"></p>
  
  <p>And after:
  <img src="https://i.stack.imgur.com/tAvTK.png" alt="enter image description here"></p>
  
  <p>I would like to avoid to insert manually the limits in the y-axis.</p>
  
  <p>So why does this happen, and how to fix it?</p>