• process.py is a script to process various files from the RATP as well as an experiment (not so successful) to automatically join stations
• index.html is the main html page, with the simple d3.js code to produce the map
• helpers.js contains a lot of functions used when generating the map
• mouse.helpers.js contains stuff to interact with the map

You need to download the files "Trafic annuel entrant par station" and "Positions géographiques des stations" from the RATP opendata site (links available from index.html source).