Not sure I understand the question... are you suggesting the concept of using vetted libraries is somehow faulty? Anyway, the answer is generally "a large community" or "some big company like google or facebook."
I think GP was referring to a situation where no such library exists or the needed modification would be prohibitive. Not that uncommon scenario once you are inventing new kinds of visualizations. Libs like d3js help, but they are pretty low level.
People knowing about algorithm+implementation details? Definitely not front-end developers. (Not trying to implicate that front-end devs can't do it... you get the point)
I don't really get the point. If you're writing an app to do X, and you require functionality Y for your interface, if a library doesn't exist, or is just some random persons GitHub dump minidress and no other committees, what do you do?