The matplotlib axes that the visualizer draws upon (can also be a grid
        of multiple axes objects). The visualizer automatically creates an
        axes for the user if one has not been specified.

def ax(self):
        """
        The matplotlib axes that the visualizer draws upon (can also be a grid
        of multiple axes objects). The visualizer automatically creates an
        axes for the user if one has not been specified.
        """
        if not hasattr(self, "_ax") or self._ax is None:
            self._ax = plt.gca()
        return self._ax