code: def trail(self):
          
          events = self.drag_events()
          if events is None:
              return None
  
          trail = np.empty((len(events), 2), dtype=int)
          for i, ev in enumerate(events):
              trail[i] = ev.pos
  
          return trail
docs: Return an (N, 2) array of mouse coordinates for every event in the
          current mouse drag operation.
  
          Returns None if there is no current drag operation.