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mapping data & grid (MAP_SET MAP_GRID)

Hi all,
   i'm making global images, with grids, according to the following

1. draw the globe on the screen so that the _geomagnetic_ pole is
2. draw the grids according to the _geographic_ poles.

The code i use is the following:
;;; data is a 2d array, sorted on latitude and longitude.
;;; the geomagnetic pole in the north is at (82, -90), for 1999.
MAP_SET, 82, -90, /ortho
img = MAP_PATCH(data, xstart=xx, ystart=yy, lon0=lon0, lon1=lon1)
TV, img, xx, yy
MAP_GRID   		;;; <--- ERROR!!

This map_grid puts the grid centered on the pole of the data. The pole i
want to center it on is different, due to the different coordinate

A new map_set to change the coordinates will not work, because the
map_grid erases the screen after a map_set

I have tried plotting to the Z-buffer, reading it, and re-displaying it to
X, but TV does not have a NO_ERASE keyword as far as i can tell, so the
data part of the image gets erased again.