def _increment_index(self, di=1):
        """
        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.
        """
        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)
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.