def plot_brillouin(self):
        """
        plot the Brillouin zone
        """

        # get labels and lines
        labels = {}
        for k in self._bs.kpoints:
            if k.label:
                labels[k.label] = k.frac_coords

        lines = []
        for b in self._bs.branches:
            lines.append([self._bs.kpoints[b['start_index']].frac_coords,
                          self._bs.kpoints[b['end_index']].frac_coords])

        plot_brillouin_zone(self._bs.lattice_rec, lines=lines, labels=labels)
plot the Brillouin zone