There's a great graphing app already included in OS X. It's called Grapher and is in the Applications > Utilities folder.
I`d like to be able to graph linear-algebraic functions in Mac OS 10.4. The discussion here seems to be centring on MatLab and the like, whereas I don`t need to do anything too fancy; in fact I`d imagine that there would be free software that does what I need. Any recommendations would be much appreciated.