docs: def histogram_relative_line_load(self, timestep=None, title=True,
                                       voltage_level='mv_lv', **kwargs):
          """
          Plots histogram of relative line loads.
  
          For more information see :func:`edisgo.tools.plots.histogram`.
  
          Parameters
          ----------
          Parameters
          ----------
          timestep : :pandas:`pandas.Timestamp<timestamp>` or None, optional
              Specifies time step histogram is plotted for. If timestep is None
              all time steps voltages are calculated for are used. Default: None.
          title : :obj:`str` or :obj:`bool`, optional
              Title for plot. If True title is auto generated. If False plot has
              no title. If :obj:`str`, the provided title is used. Default: True.
          voltage_level : :obj:`str`
              Specifies which voltage level to plot voltage histogram for.
              Possible options are 'mv', 'lv' and 'mv_lv'. 'mv_lv' is also the
              fallback option in case of wrong input. Default: 'mv_lv'
  
          """
          residual_load = tools.get_residual_load_from_pypsa_network(
              self.network.pypsa)
          case = residual_load.apply(
              lambda _: 'feedin_case' if _ < 0 else 'load_case')
          if timestep is not None:
              timeindex = [timestep]
          else:
              timeindex = self.network.results.s_res().index
          load_factor = pd.DataFrame(
              data={'s_nom': [float(self.network.config[
                                        'grid_expansion_load_factors'][
                                        'mv_{}_line'.format(case.loc[_])])
                              for _ in timeindex]},
              index=timeindex)
          if voltage_level == 'mv':
              lines = self.network.pypsa.lines.loc[
                  self.network.pypsa.lines.v_nom > 1]
          elif voltage_level == 'lv':
              lines = self.network.pypsa.lines.loc[
                  self.network.pypsa.lines.v_nom < 1]
          else:
              lines = self.network.pypsa.lines
          s_res = self.network.results.s_res().loc[
              timeindex, lines.index]
          # get allowed line load
          s_allowed = load_factor.dot(
              self.network.pypsa.lines.s_nom.to_frame().T * 1e3)
          # get line load from pf
          data = s_res.divide(s_allowed)
  
          if title is True:
              if timestep is not None:
                  title = "Relative line load histogram for time step {}".format(
                      timestep)
              else:
                  title = "Relative line load histogram \nfor time steps " \
                          "{} to {}".format(data.index[0], data.index[-1])
          elif title is False:
              title = None
          plots.histogram(data=data, title=title, **kwargs)