"GitHub is a specific community that’s grown very quickly since it launched [writeup]. It was not initially reflective of open source as a whole but rather centered around the Ruby on Rails community"

Github has moved out of a niche over the past five years, and the graph demonstrates that.

Not really. It is universal that statistics can easily misrepresent if one doesn't take a critical look at choice of data and how it is presented. For example in this case the focus on new repos/issues/etc. is suspicious to me. It could very well be that we just see a rise in one-off Javascript uploads to Github here, instead of a comparison of sustained development on mature projects.