Where/how can I find out what’s the approximate total network graphs per second? Thanks

Or (how) is network difficulty related to graphs per second in some easy way?

Difficulty is graphs per second times 10 / 7 (one of every 42 graphs in 60 seconds has a 42-cycle).

So if grin(miner) says that network difficulty is 1, 2, or 3 it means that total network graphs per second is around 7/10, 14/10 or 21/10?

Usually quite a bit more, since there’s a repeated truncation in the difficulty computation. Once that’s fixed, a difficulty of 3 indeed means that gps is between 2.1 and 2.8

Shameless self-plug: you can view the network graph rate of the last 7 days at https://grinscan.net/charts