Called per artist (group), with possibly a list of indices.

def onpick(self, event):
        """Called per artist (group), with possibly a list of indices."""
        if hasattr(event.artist, '_mt_legend_item'):
            # legend item, instead of data point
            idx = event.artist._mt_legend_item
            try:
                self.toggle_artist(self.artists[idx])
            except IndexError:
                pass
            return

        # only print logevents of visible points
        if not event.artist.get_visible():
            return

        # get PlotType and let it print that event
        plot_type = event.artist._mt_plot_type
        plot_type.clicked(event)