Once a graph has been displayed, several other contour images can be
overplotted on it, using ISOCONT with the /OVERPLOT keyword several
times. Note that in this case, the legend created by the /PUTINFO
keyword is not updated. Note also that ISOCONT must be used at least
once without the /OVERPLOT keyword.
Example:
1. [ ISOCONT, 'm51_dss.fits', iso_struct_raster ] and then
2. [ ISOCONT, 'm51_dss.fits', new_raster, /OVERPLOT ]
The /OVERPLOT enables a user to display contour levels of the new_raster over the previous output of ISOCONT. This is useful if one wants to superimpose contour maps of several observations of the same sky region.