[johnmmurray]

So.. What exactly does the author of xkcd use? Is it a secret or something?

[yolesaber]

I assume Randall draws them by hand. The humor / inventiveness of this question consists of trying to replicate the hand-drawn style using code.

[lukeschlather]

He doesn't specifically talk about graphs, but he says his usual workflow is sketch in pencil, then draw, then scan and postprocess:

http://www.maa.org/Mathhorizons/MH-Sep2012_XKCD.html