title: How to add a second x-axis in matplotlib
body: <p>I have a very simple question. I need to have a second x-axis on my plot and I want that this axis has a certain number of tics that correspond to certain position of the first axis.</p>
  
  <p>Let's try with an example. Here I am plotting the dark matter mass as a function of the expansion factor, defined as 1/(1+z), that ranges from 0 to 1.</p>
  
  <pre><code>semilogy(1/(1+z),mass_acc_massive,'-',label='DM')
  xlim(0,1)
  ylim(1e8,5e12)
  </code></pre>
  
  <p>I would like to have another x-axis, on the top of my plot, showing the corresponding z for some values of the expansion factor. Is that possible? If yes, how can I have xtics ax</p>