Standard plot of spectrum

        Parameters
        ----------
        title: string|None, optional
            Title of plot
        reciprocal: sip_response object|None, optional
            If provided, plot this spectrum with another color
        limits: dict|None, optional
            used to set ylimits of the plots. Possible entries: rmag_min,
            rmag_max, rpha_min, rpha_max, cre_min, cre_max, cim_min, cim_max
        dtype: string, optional
            Possible values: [rho|R]. Determines the label types. 'rho':
                resistivity/conductivity, 'r': resistance/conductance
        label_nor:
            label for normal data (default: "normal")
        label_rec:
            label for reciprocal data (default: "reciprocal")

        Returns
        -------
        fig: figure object
            the generated matplotlib figure
        axes: list
            matplotlib axes objects

def _plot(self, title=None, reciprocal=None, limits=None, dtype='rho',
              **kwargs):
        """Standard plot of spectrum

        Parameters
        ----------
        title: string|None, optional
            Title of plot
        reciprocal: sip_response object|None, optional
            If provided, plot this spectrum with another color
        limits: dict|None, optional
            used to set ylimits of the plots. Possible entries: rmag_min,
            rmag_max, rpha_min, rpha_max, cre_min, cre_max, cim_min, cim_max
        dtype: string, optional
            Possible values: [rho|R]. Determines the label types. 'rho':
                resistivity/conductivity, 'r': resistance/conductance
        label_nor:
            label for normal data (default: "normal")
        label_rec:
            label for reciprocal data (default: "reciprocal")

        Returns
        -------
        fig: figure object
            the generated matplotlib figure
        axes: list
            matplotlib axes objects
        """
        if limits is None:
            limits = {}

        fig, axes = plt.subplots(
            2, 2, figsize=(15 / 2.54, 6 / 2.54), sharex=True
        )
        if title is not None:
            fig.suptitle(title)

        # resistivity magnitude
        if limits is None:
            limits = {}

        ax = axes[0, 0]
        ax.semilogx(
            self.frequencies, self.rmag, '.-', color='k',
            label=kwargs.get('label_nor', 'normal'),
        )
        ax.set_ylim(
            limits.get('rmag_min', None),
            limits.get('rmag_max', None)
        )

        # resistivity phase
        ax = axes[0, 1]
        ax.semilogx(self.frequencies, -self.rpha, '.-', color='k')
        # note the switch of _min/_max because we change the sign while
        # plotting
        ymin = limits.get('rpha_max', None)
        if ymin is not None:
            ymin *= -1
        ymax = limits.get('rpha_min', None)
        if ymax is not None:
            ymax *= -1
        ax.set_ylim(
            ymin,
            ymax,
        )

        # conductivity real part
        ax = axes[1, 0]
        ax.loglog(self.frequencies, self.cre, '.-', color='k')
        ax.set_ylim(
            limits.get('cre_min', None),
            limits.get('cre_max', None)
        )

        # conductivity imaginary part
        ax = axes[1, 1]
        ax.loglog(self.frequencies, self.cim, '.-', color='k')
        ax.set_ylim(
            limits.get('cim_min', None),
            limits.get('cim_max', None)
        )

        self._add_labels(axes, dtype)

        for ax in axes.flatten()[0:2]:
            ax.xaxis.set_major_locator(mpl.ticker.LogLocator(numticks=5))
            ax.yaxis.set_major_locator(mpl.ticker.MaxNLocator(5))

        for ax in axes.flatten()[2:]:
            ax.xaxis.set_major_locator(mpl.ticker.LogLocator(numticks=5))
            ax.yaxis.set_major_locator(mpl.ticker.LogLocator(numticks=5))

        fig.tight_layout()
        # plot reciprocal spectrum
        if reciprocal is not None:
            axes[0, 0].semilogx(
                reciprocal.frequencies,
                reciprocal.rmag,
                '.-',
                color='k',
                linestyle='dashed',
                label=kwargs.get('label_rec', 'reciprocal'),
            )
            axes[0, 1].semilogx(
                reciprocal.frequencies,
                -reciprocal.rpha,
                '.-',
                color='k',
                linestyle='dashed',
            )
            axes[1, 0].loglog(
                reciprocal.frequencies,
                reciprocal.cre,
                '.-',
                color='k',
                linestyle='dashed',
            )
            axes[1, 1].loglog(
                reciprocal.frequencies,
                reciprocal.cim,
                '.-',
                color='k',
                linestyle='dashed',
            )

            fig.subplots_adjust(
                bottom=0.3,
            )

            axes[0, 0].legend(
                loc="lower center",
                ncol=4,
                bbox_to_anchor=(0, 0, 1, 1),
                bbox_transform=fig.transFigure,
                fontsize=7.0,
            )

        return fig, axes