title: Reading values from a graph in Python
body: <p>I'm currently trying to plot data from a SPICE simulation in python. I have succeed in plotting the information but now I require to extract values from specific points, say for example I would like to find the <code>x</code> value for a given <code>y</code> or vice versa.</p>
  
  <pre><code>pylab.figure(1)
  pylab.title('NMOS')
  pylab.semilogy(row.ngm_id[81*0:81*1],row.nidw[81*0:81*1],label='L = 0.35u')
  pylab.xlabel('gm/ID [S/A]',fontsize=20)
  pylab.ylabel('ID/W [A/m]',fontsize=20)
  pylab.legend()
  pylab.grid()
  pylab.show()
  </code></pre>