set xlevs lev1 lev2 ... levN
Allows the user to specify each individual labeled tick mark for the X-axis.
set xlevs will override the tick mark interval specified with
the set xlint command.
set xaxis to specify
labeled tick marks and also invoke set xlevs, then the tick marks
lev1 ... levN specified with set xlevs will
appear if they fall within the start and end
range specified in the set xaxis command.