docs: def plot_rpha(self, filename, pmin=None, pmax=None, title=None):
          """plot all resistance/resistivity phase spectra
          """
          cmap = mpl.cm.get_cmap('viridis')
          SM = mpl.cm.ScalarMappable(norm=None, cmap=cmap)
          colors = SM.to_rgba(np.linspace(0, 1, len(self.objects)))
          fig, ax = plt.subplots(1, 1, figsize=(15 / 2.54, 15 / 2.54))
          for nr, item in enumerate(self.objects):
              ax.semilogx(
                  item.frequencies,
                  -item.rpha,
                  '.-',
                  color=colors[nr],
                  label=self.labels[nr],
              )
          ax.set_xlim(*self.xlim)
          ax.set_ylabel(sip_labels.get_label('rpha', 'meas', 'mathml'))
          ax.set_xlabel('frequency [Hz]')
          ax.set_ylim(pmin, pmax)
          if title is not None:
              ax.set_title(title)
          self._add_legend(ax)
          fig.tight_layout()
          fig.subplots_adjust(bottom=0.5)
          fig.savefig(filename, dpi=300)
          plt.close(fig)