Label the x axis on the bottom row of the grid.

def set_xlabels(self, label=None, **kwargs):
        """Label the x axis on the bottom row of the grid."""
        if label is None:
            label = label_from_attrs(self.data[self._x_var])
        for ax in self._bottom_axes:
            ax.set_xlabel(label, **kwargs)
        return self