def _add_overlay(self, center):
        """
        Add a grid from a differently-centered stereonet. This is useful for
        making "polar stereonets" that still use the same coordinate system as
        a standard stereonet.  (i.e. a plane/line/whatever will have the same
        representation on both, but the grid is displayed differently.)

        To display a polar grid on a stereonet, use ``kind="polar"``.

        It is also often useful to display a grid relative to an arbitrary
        measurement (e.g. a lineation axis).  In that case, use the
        ``lon_center`` and ``lat_center`` arguments.  Note that these are in
        radians in "stereonet coordinates".  Therefore, you'll often want to
        use one of the functions in ``stereonet_math`` to convert a
        line/plane/rake into the longitude and latitude you'd input here. For
        example:  ``add_overlay(center=stereonet_math.line(plunge, bearing))``.

        If no parameters are specified, this is equivalent to turning on the
        standard grid.

        Parameters
        ----------
        center: 2-item tuple of numbers
            A tuple of (longitude, latitude) in radians that the overlay is
            centered on.
        """
        plunge, bearing = stereonet_math.geographic2plunge_bearing(*center)
        lon0, lat0 = center
        fig = self.get_figure()
        self._overlay_axes = fig.add_axes(self.get_position(True),
                                          frameon=False, projection=self.name,
                                          center_longitude=0,
                                          center_latitude=np.radians(plunge),
                                          label='overlay',
                                          rotation=bearing)
        self._overlay_axes._polar.remove()
        self._overlay_axes.format_coord = self._overlay_format_coord
        self._overlay_axes.grid(True)