It was done in-house. We really care about how our graphs look, because graphs represent information. It's like typography. Humans learned how to write 5000 years ago, and it took us a very long while until we mastered the art of transmitting information like that. Once we mastered it, however, we started caring about the way we present that information. For no specific reason, but we developed unique typefaces that look beautiful, different, yet transmit the same information.
Likewise, we've spent a ton of valuable time graphing and mining data from all over our stack; it's only natural that after that, we care about how we present that data. Like typography, it's hard because it's an arduous hunt for the aesthetics hidden behind information. But this hunt is what makes us human.
That's why we design beautiful graphs, just like we'd design a beautiful typeface. Not because we are GitHubbers, nor because we are designers, but because we are humans.
Life is too short to stare at ugly graphs. And yes, you should feel bad.
PS: we have no project managers at GitHub.
An oldie, but I'm still loving it (emphasis mine).