I found this thread
on google, which discusses the relative merits of scientific plotting software in linux. The poster highly recommends a combination of gnumeric
(a gnome-based spreadsheet, that my accountant wife says is comparable to Excel XP) and guppi
(gnome-based scientific plotting tool).
The guppi project page also has links
to other scientific plotting software.
You may also want to check out SciGraphica
, another GPL scientific plotting package.