I assume it's just getting a lot of traffic from HN and github have some limits in place to stop it being abused for content hosting. I imagine it'll start working again once the traffic goes back to normal.
I take it this is the code to generate the images, mention was made of a data file, but it doesn't appear to be in the repo, from a brief check I didn't see any reference to a data file.
I am guessing the actual data file itself would be smaller than all of the images. HN readers should have a high chance of being able to run the code.