I use like many others the free software GNU Octave for my numerical computations. Just like Scilab, Octave serves as a good free alternative to the commercial software Matlab. I prefer Octave over Scilab because it belongs to the GNU Project and because its syntax is more compatible with the one of Matlab (official objective of Octave). Scilab is quite popular in France for various reasons. Actually, one can also use the more recent Python packages matplotlib, SciPy, NumPy. You may browse the online matplotlib gallery. The installation of matplotlib/SciPy/NumPy is fearly easy on a Debian GNU/Linux system:
sudo apt-get install python-matplotlib ipython
If you do not know it yet, iphyton is “an interactive shell for the Python programming language that offers enhanced introspection, additional shell syntax, tab completion and rich history”.