ughhh
excel is NOT meant for stats or graphs! neither is gnumeric, but they have at least fixed the problems that excel has with precision. Microsoft has ignored this problem for over a decade.
Free and a million times better than excel:
http://www.r-project.org/
http://plplot.sourceforge.net/
http://www.gnu.org/software/octave/
http://www.scilab.org/
and for graphing/fitting basic stuff in a more familiar way to excel users (uses tables) use qti-plot:
http://soft.proindependent.com/qtiplot.html
it's free (gratis) for linux and cheap for mac osx/windows. It's open source so you could install it free on mac/windows but I would just pay for the precompiled binary.
Sci Davis is similar (but more limited in functionality) but free for all platforms.
If you want to waste time and money using excel, then go for it. I think that gnumeric or libre office are both powerful enough for what these programs are built for. I use libre office to keep track of my marks and my bills. That's what these programs are designed to do. To quickly set up a sheet to calculate stuff. It's not the proper tool for doing real work.
Take the time to learn the proper software and soon enough it will pay off and you will save lots of time in the long run.
It's the exact same for like ms word.
Sure it might be ok for like a term paper, but imagine how much of a headache it would be to write your masters thesis in word! You might as well learn LaTeX now (or start with LyX).