They're quite different, though, and I can see why many prefer ggplot. It's a declarative, domain-specific language that implements a Tufte-inspired "grammar of graphics" (hence the gg- in the name; see section 1.3 of [1], and [2,3]) for very fast and convenient interactive plotting, whereas matplotlib is just a clone of MATLIB's procedural plotting API.
> To be honest, matplotlib seems a good contender to me (http://matplotlib.org/).
They're quite different, though, and I can see why many prefer ggplot. It's a declarative, domain-specific language that implements a Tufte-inspired "grammar of graphics" (hence the gg- in the name; see section 1.3 of [1], and [2,3]) for very fast and convenient interactive plotting, whereas matplotlib is just a clone of MATLIB's procedural plotting API.
[1] http://www.amazon.com/ggplot2-Elegant-Graphics-Data-Analysis...
[2] http://www.amazon.com/The-Grammar-Graphics-Statistics-Comput...
[3] http://vita.had.co.nz/papers/layered-grammar.html