Syntax: CONTOUR

   Makes a contour plot of an image read by the IMAGE command (q.v.).
The contour levels are set using the LEVELS command. Contrary to
previous versions of the CONTOUR command, plot coordinates are taken
to be those set by the LIMITS command.

   Any pixels whose value is NaN are ignored, as are any whose value
equals the SM variable $missing_data; by default this is some very
large number.