AITOFF_GRID

       Produce an overlay of latitude and longitude lines over a plot or image

       The grid is plotted on the current graphics device. AITOFF_GRID
       assumes that the ouput plot coordinates span the x-range of
       -180 to 180 and the y-range goes from -90 to 90.

       AITOFF_GRID [,DLONG,DLAT, LABEL=, /NEW, CHARTHICK=, CHARSIZE=, _EXTRA=]

       DLONG   = Optional input longitude line spacing in degrees. If left
                 out, defaults to 30.
       DLAT    = Optional input latitude line spacing in degrees. If left
                 out, defaults to 30.

       LABEL           = Optional keyword specifying that the latitude and
                         longitude lines on the prime meridian and the
                         equator should be labeled in degrees. If LABELS is
                         given a value of 2, i.e. LABELS=2, then the longitude
                         labels will be in hours instead of degrees.
        CHARSIZE       = If /LABEL is set, then CHARSIZE specifies the size
                         of the label characters (passed to XYOUTS)
        CHARTHICK     =  If /LABEL is set, then CHARTHICK specifies the
                         thickness of the label characters (passed to XYOUTS)
       /NEW          =   If this keyword is set, then AITOFF_GRID will create
                         a new plot grid, rather than overlay an existing plot.
       Any valid keyword to OPLOT such as COLOR, LINESTYLE, THICK can be
       passed to AITOFF_GRID (though the _EXTRA facility) to to specify the
       color, style, or thickness of the grid lines.

       Draws grid lines on current graphics device.

       Create a labeled Aitoff grid of the Galaxy, and overlay stars at
       specified Galactic longitudes, glong and latitudes, glat
       IDL> aitoff_grid,/label,/new        ;Create labeled grid
       IDL> aitoff, glong, glat, x,y      ;Convert to X,Y coordinates
       IDL> plots,x,y,psym=2              ;Overlay "star" positions

       AITOFF

       If labeling in hours (LABEL=2) then the longitude spacing should be
       a multiple of 15 degrees

       J. Bloch        1.2     6/2/91
       Converted to IDL V5.0   W. Landsman   September 1997
       Create default plotting coords, if needed   W. Landsman  August 2000
       Added _EXTRA, CHARTHICK, CHARSIZE keywords  W. Landsman  March 2001
       Several tweaks, plot only hours not minutes W. Landsman January 2002

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