code: def _add_overlay(self, center):
          
          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)
docs: 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.