code: def click_estimate_curvature():
      
  
      c1 = _pylab.ginput()
      if len(c1)==0:
          return None
  
      c2 = _pylab.ginput()
      if len(c2)==0:
          return None
  
      return 2*(c2[0][1]-c1[0][1])/(c2[0][0]-c1[0][0])**2
docs: Takes two clicks and returns the curvature, assuming the first click
      was the minimum of a parabola and the second was some other point.
  
      Returns the second derivative of the function giving this parabola.
  
      Right-click aborts.