code: def click_estimate_difference():
      
  
      c1 = _pylab.ginput()
      if len(c1)==0:
          return None
  
      c2 = _pylab.ginput()
      if len(c2)==0:
          return None
  
      return [c2[0][0]-c1[0][0], c2[0][1]-c1[0][1]]
docs: Takes two clicks and returns the difference vector [dx, dy].
  
      Right-click aborts.