right button down: show pop-up

def onRightDown(self, event=None):
        """ right button down: show pop-up"""
        if event is None:
            return
        # note that the matplotlib event location have to be converted
        if event.inaxes is not None and self.popup_menu is not None:
            pos = event.guiEvent.GetPosition()
            wx.CallAfter(self.PopupMenu, self.popup_menu, pos)
        self.cursor_mode_action('rightdown', event=event)
        self.ForwardEvent(event=event.guiEvent)