title: Can matplotlib pick_event return array indeces rather than values or pixels?
body: <p>I'm trying to write a point picker for a plot in matplotlib. I'd like it to tell me the i, j indeces of the plotted array where I have clicked. It seems that the only options are for event.mouseevent.xdata, which is in terms of the plotted coordinate values, or event.mouseevent.x, which is in terms of pixels (from what I can see "pixels" isn't the same as grid boxes i.e. what I'm looking for).</p>
  
  <p>Have I misunderstood what xdata/x are. Is there a way to do what I want.</p>
  
  <p>Thanks
  Niall</p>
  
  <p>EDIT: the image being clicked on is a 2D array plot, for instance with pcolormesh</p>