Seems like something really weird going on here.

Either a very accurately controlled farm based upon some sort of off-peak electricity price plan, or something else...
I have seen this pattern before in corporate IT traffic: the rise in the morning, the gap at noon for lunch, rise again in the afternoon.... and decline. From Monday to Friday.
A botnet?
Hmm, but it makes no sense to hash more when computer is used more (asuming that increased traffic causes more CPU use). Graph should
be inverse of pattern you mentioned, e.g. flip the image vertically. Or is it that miner tracks CPU usage in such a way to not cause CPU and
traffic increase at usualy idle times, suspicious behaviour easily detectable by sysadmins?
Yes, assuming a desktop/laptop 24 hours on. But to save power most companies don't allow their workers to leave the computers on. That's why I said it looks like IT corporate traffic.
Intriguing.