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.