code: def plot_zt_mu(self, temp=600, output='eig', relaxation_time=1e-14,
                     xlim=None):
          
          import matplotlib.pyplot as plt
          plt.figure(figsize=(9, 7))
          zt = self._bz.get_zt(relaxation_time=relaxation_time, output=output,
                               doping_levels=False)[temp]
          plt.plot(self._bz.mu_steps, zt, linewidth=3.0)
          self._plot_bg_limits()
          self._plot_doping(temp)
          if output == 'eig':
              plt.legend(['ZT$_1$', 'ZT$_2$', 'ZT$_3$'])
          if xlim is None:
              plt.xlim(-0.5, self._bz.gap + 0.5)
          else:
              plt.xlim(xlim)
          plt.ylabel("ZT", fontsize=30.0)
          plt.xlabel("E-E$_f$ (eV)", fontsize=30.0)
          plt.xticks(fontsize=25)
          plt.yticks(fontsize=25)
          plt.tight_layout()
          return plt
docs: Plot the ZT in function of Fermi level.
  
          Args:
              temp: the temperature
              xlim: a list of min and max fermi energy by default (0, and band
                  gap)
              tau: A relaxation time in s. By default none and the plot is by
                 units of relaxation time
  
          Returns:
              a matplotlib object