Update the toolbar menu - e.g., called when a new subplot
        or axes are added

def update(self):
        """
        Update the toolbar menu - e.g., called when a new subplot
        or axes are added
        """
        DEBUG_MSG("update()", 1, self)
        self._axes = self.canvas.figure.get_axes()
        self._menu.updateAxes(len(self._axes))