code: def _increment_index(self, di=1):
          
          if self._last_event is None:
              return
  
          if not hasattr(self._last_event, 'ind'):
              return
  
          event = self._last_event
          xy = pick_info.get_xy(event.artist)
  
          if xy is not None:
              x, y = xy
              i = (event.ind[0] + di) % len(x)
              event.ind = [i]
              event.mouseevent.xdata = x[i]
              event.mouseevent.ydata = y[i]
  
              self.update(event, self._last_annotation)
docs: Move the most recently displayed annotation to the next item in the
          series, if possible. If ``di`` is -1, move it to the previous item.