R is the language of choice for statisticians. That means just about any statistical technique you can imagine is available as a library in R. It is very nice as an interactive environment to explore your data. The fact that it was designed by stats people also leads to some funky language choices. There are actually a number of other graphics packages in addition to ggplot. Good integration with C for speed (it can be slow otherwise on iterative code, and a bit of a memory hog, as it copies objects in memory with reckless abandon.)

It is my second choice language these days after python, and the only one to use when you need a certain library.