Get my plane axes indexes: these are already
        (as compared with higher level planes),
        as we are returning a DataFrame axes.

def _get_plane_axes(self, axis):
        """
        Get my plane axes indexes: these are already
        (as compared with higher level planes),
        as we are returning a DataFrame axes.
        """
        return [self._get_axis(axi)
                for axi in self._get_plane_axes_index(axis)]