Create a multi-plot with one pseudosection for each frequency.

        Parameters
        ----------
        column : string
            which column to plot
        filename : None|string
            output filename. If set to None, do not write to file. Default:
            None
        return_fig : bool
            if True, return the generated figure object. Default: False

        Returns
        -------
        fig : None|matplotlib.Figure
            if return_fig is set to True, return the generated Figure object

def plot_pseudosections(self, column, filename=None, return_fig=False):
        """Create a multi-plot with one pseudosection for each frequency.

        Parameters
        ----------
        column : string
            which column to plot
        filename : None|string
            output filename. If set to None, do not write to file. Default:
            None
        return_fig : bool
            if True, return the generated figure object. Default: False

        Returns
        -------
        fig : None|matplotlib.Figure
            if return_fig is set to True, return the generated Figure object
        """
        assert column in self.data.columns

        g = self.data.groupby('frequency')
        fig, axes = plt.subplots(
            4, 2,
            figsize=(15 / 2.54, 20 / 2.54),
            sharex=True, sharey=True
        )
        for ax, (key, item) in zip(axes.flat, g):
            fig, ax, cb = PS.plot_pseudosection_type2(
                item, ax=ax, column=column
            )
            ax.set_title('f: {} Hz'.format(key))
        fig.tight_layout()
        if filename is not None:
            fig.savefig(filename, dpi=300)

        if return_fig:
            return fig
        else:
            plt.close(fig)