You ask about the Y2K compliance of gnuplot.  Gnuplot is freeware authored
by a collection of volunteers, who cannot make any legal statement about
the compliance or non-compliance of gnuplot or its uses.  That said, the
following information may be helpful:

Gnuplot's compliance depends in part on the compliance of the underlying
operating system and hardware.  The only use gnuplot makes of a system-
supplied date is in the "set timestamp" command, which simply echos the date
on the plot.  If the underlying OS cannot produce an accurate time string,
then the "set timestamp" command may fail to print the correct date on plots.

In gnuplot 3.5, if the user chooses to use %y in a timestamp format, rather
than %Y, it will print 2-digit rather than 4-digit years.  The effects depend
on the importance you place on the timestamps printed on plots.

gnuplot 3.7 also allows the use of time/date data as variables, but the user
has complete control over the input format of the data and the output format
of the tic labels -- the same 2-digit "%y" (interpreted as 1900+) and 4-digit
"%Y" formats are both available.  But again, these are user-specifiable, so
if there is a Y2K problem here, it is the responsibility of the user.

IMPORTANT NOTICE
As of gnuplot beta version 3.7.0.9, the interpretation of the "%y" two
digit year specifier was changed in accordance with the recommendations of
The Open Group and all major Unix vendors.  When a century is not otherwise
specified, values in the range 69-99 refer to the twentieth century and values
in the range 00-68 refer to the twenty-first century.