The dash d command instructs plotl to use a dashed
line (the dash length being defined by d) with equal
mark and space for data curves.
The command nodash returns the data curve to the default,
The command pair, mark dm and space ds determine
the mark and space of a dash. For example, issuing the commands
would cause data curves to be dashed with pend down for 0.1 inch, and
pen up for 0.5 inch.