Especially when you have such pretty defaults in things like Matlab and Mathematica.
Matplotlib (http://matplotlib.sourceforge.net/) is actually the best opensource library for creating great looking graphs that I have come across, and is comparable to Matlab and Mathematica.
I guess taste comes in place - I find producing good (as in academic publication good) figures in matlab an exercice of pain. The subplot mechanism is awful (at least was 5 years ago), and it is hard to control the layout. I heard mathematic is much better in that regard, but never used it myself.
Matplotlib (http://matplotlib.sourceforge.net/) is actually the best opensource library for creating great looking graphs that I have come across, and is comparable to Matlab and Mathematica.