We are using MadAnalysis 5, a high-energy physics package dedicated to the analysis of simulated collisions (see Ep. 3). This code takes care of not only the analysis of the simulated collisions, but also of handling the plots.
For the figures shown in the present blog, MadAnalysis 5 relies on matplotlib. Note that ROOT can also be optionally used (this is what @travelingmercies uses).
PS: You can join us on this citizen science adventure on Hive if you want to try this program ;)