code: def click_estimate_slope():
      
  
      c1 = _pylab.ginput()
      if len(c1)==0:
          return None
  
      c2 = _pylab.ginput()
      if len(c2)==0:
          return None
  
      return (c1[0][1]-c2[0][1])/(c1[0][0]-c2[0][0])
docs: Takes two clicks and returns the slope.
  
      Right-click aborts.