CONTOUR:contour_level_selection
Syntax: CONTOUR\SPECIFIC { x y } v lvls
Specific contour levels can be requested by using the \SPECIFIC
qualifier. Vector lvls should contain the desired contour levels.
By default, the entire range of possible contour levels will be used
to determine the minimum and maximum contour levels. If the \PARTIAL
qualifier is used, the minimum and maximum contour levels will be
determined by the region contained within the axes. To zoom in on a
particular region for more detail, pre-set the axis scales using the
SCALES command.