Link to the data explorer : https://manpopex.us/
I came across this visualization on reddit and thought i share it here. Justin Fung has provided more information on the reddit feed as well as https://manpopex.us/. Seems to me that the data is sourced from MTA website. I have accessed the data provided by MTA on turnstile usage at the subway stops. More information on the data is available here.
The author uses R and Mapbox to generate and clean the data.