Plot the seebeck coefficient in function of Fermi level

        Args:
            temp:
                the temperature
            xlim:
                a list of min and max fermi energy by default (0, and band gap)
        Returns:
            a matplotlib object

def plot_seebeck_mu(self, temp=600, output='eig', xlim=None):
        """
        Plot the seebeck coefficient in function of Fermi level

        Args:
            temp:
                the temperature
            xlim:
                a list of min and max fermi energy by default (0, and band gap)
        Returns:
            a matplotlib object
        """
        import matplotlib.pyplot as plt
        plt.figure(figsize=(9, 7))
        seebeck = self._bz.get_seebeck(output=output, doping_levels=False)[
            temp]
        plt.plot(self._bz.mu_steps, seebeck,
                 linewidth=3.0)

        self._plot_bg_limits()
        self._plot_doping(temp)
        if output == 'eig':
            plt.legend(['S$_1$', 'S$_2$', 'S$_3$'])
        if xlim is None:
            plt.xlim(-0.5, self._bz.gap + 0.5)
        else:
            plt.xlim(xlim[0], xlim[1])
        plt.ylabel("Seebeck \n coefficient  ($\\mu$V/K)", fontsize=30.0)
        plt.xlabel("E-E$_f$ (eV)", fontsize=30)
        plt.xticks(fontsize=25)
        plt.yticks(fontsize=25)
        plt.tight_layout()
        return plt