def _add_labels(self, axes, dtype):
        """Given a 2x2 array of axes, add x and y labels

        Parameters
        ----------
        axes: numpy.ndarray, 2x2
            A numpy array containing the four principal axes of an SIP plot
        dtype: string
            Can be either 'rho' or 'r', indicating the type of data that is
            plotted: 'rho' stands for resistivities/conductivities, 'r' stands
            for impedances/condactances

        Returns
        -------
        None
        """
        for ax in axes[1, :].flat:
            ax.set_xlabel('frequency [Hz]')

        if dtype == 'rho':
            axes[0, 0].set_ylabel(r'$|\rho| [\Omega m]$')
            axes[0, 1].set_ylabel(r'$-\phi [mrad]$')
            axes[1, 0].set_ylabel(r"$\sigma' [S/m]$")
            axes[1, 1].set_ylabel(r"$\sigma'' [S/m]$")
        elif dtype == 'r':
            axes[0, 0].set_ylabel(r'$|R| [\Omega]$')
            axes[0, 1].set_ylabel(r'$-\phi [mrad]$')
            axes[1, 0].set_ylabel(r"$Y' [S]$")
            axes[1, 1].set_ylabel(r"$Y'' [S]$")
        else:
            raise Exception('dtype not known: {}'.format(dtype))