plot all resistance/resistivity phase spectra

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)