GMT is a collection of public-domain Unix tools that allows you to
manipulate x,y and x,y,z data sets (filtering, trend fitting,
gridding, projecting, etc.) and produce PostScript illustrations
ranging from simple x-y plots, via contour maps, to artificially
illuminated surfaces and 3-d perspective views in black/white or
24bit color. Linear, log10, and power scaling is supported in
addition to 25 common map projections. The processing and display
routines within GMT are completely general and will handle any (x,y)
or (x,y,z) data as input.

This port installs only the GMT manpages, there is a tutorial and
documentation in .ps, .pdf and .html format on the ftp site, too.

In case you look for data to plot, there is topological data at
ftp://topex.ucsd.edu/pub/global_topo_2min/topo_8.2.img
(140MB, covers nearly the whole earth)