geg is a very simple utility for parsing and plotting 2D-functions, eg f(x) = 3 + sin(x), and much more complicated functions of course. It is written with GTK+ and provides a neat, configurable user interface. Already packaged in the main debian archive.
Web : http://www.infolaunch.com/~daveb
Short presentation in French: http://logiciels-libres-cndp.ac-versailles.fr/article.php3?id_article=48