title: Get the inverse function of a polyfit in numpy
body: <p>I have fit a second order polynomial to a number of x/y points in the following way:</p>
  
  <pre><code>poly  = np.polyfit(x, y, 2)
  </code></pre>
  
  <p>How can I invert this function in python, to get the two x-values corresponding to a specific y-value?</p>