title: A second non-linear x2-axis, depending on the 1st one values
body: <p>I plotted a figure with a classical x-axis, let's say from 0 to 25.</p>
  
  <p>I'd like to have the x2-axis having the sqrt of x1-values and I didn't find anything about how to do it.</p>
  
  <p>With my values here, x2range would be [0:5]
  But tics shouldn't be linear, tha's why i didn't succeed to make it.</p>
  
  <p>x2-tic 5 would match with 25, x2-tic 4 with 16, 3 with 9, etc.</p>
  
  <p>Any Help ?
  I hope there's a not-too-hard way to do this</p>
  
  <p>Cheers.</p>