Autocorrelation of a trace



I was reading rhme3 writeup by cloakware-ctf and there is interesting autocorrelation visualization of a trace. I can’t understand how function of one variable can produce plot like that. I would expect something which is also function of one variable, and that indeed is only thing popping up when googling for autocorrelation.

Does anybody know how to reproduce it?

Here you can check my attempts it: gist