Syntax: MINMAX min max

   Set variables `min' and `max' to the the max and min values of an
image read by the IMAGE command. Only the region within the current
LIMITS is examined. This command may be useful for setting contour
levels, or doing a halftone plot (not currently implemented).

   For example, the commands:
      MINMAX min max
      SET lev = $min,$max,($max-$min)/9 LEVELS lev
set 10 levels for a CONTOUR plot.