Label the y axis on the left column of the grid.

def set_ylabels(self, label=None, **kwargs):
        """Label the y axis on the left column of the grid."""
        if label is None:
            label = label_from_attrs(self.data[self._y_var])
        for ax in self._left_axes:
            ax.set_ylabel(label, **kwargs)
        return self