NN is (was) also used in neapolitan comedy, derived from earlier use throughout the Roman Empire and Middle Age.
The "figlio di NN", or "son of NN" means someone who was found and adopted (typically by nuns) and whose parents were unknown.
NN can be used in general for people whose origin is uncertain. I think in this specific case it's a bit misleading - although the wikipedia article seems to suggest that NN can also be used as a synonym for "unknown", although from a historical perspective it is a bit incorrect.
The literal translation from Latin creates some confusion if you didn't know the context.
Another useful site for that purpose is the Online Encyclopedia of Integer Sequences (https://oeis.org/).
A pretty successful strategy for "cheating" at Project Euler is to implement a brute force solution to get the first few numbers out of a sequence and look it up in OEIS. If you're lucky, they've already listed a closed form expression for calculating the sequence at an arbitrary index. Drop the expression into your code, and you end up with something that tends to be orders of magnitude more efficient.
Obviously it depends on the structure of the exercise, but it's somewhat surprising how far it can take you.
your graph looks similar to the authors. Huge sprawling areas of meaningless changes, punctuated by actual work, which more or less was consistent throughout history.
The contribution graph is in my opinion really a great motivational tool. Seems to work better for me than projects like Julython, however I'm guilty of a few single-commit days fixing a typo or updating some dependency packages. Looking forward to 365!