Add monthly activity chart

[rr-]

I'VE GOT THE MOCKUP! Not

[oczki]

We've been discussing it on IM, come on.

The suggestions were:

• punch card
• heatmap (below), though it looks like arkanoid a bit

I've experimented with cell padding and for values higher than 1 px the optical illusion comes in, so it shouldn't be more than 1 px. I didn't check it without padding.

[oczki]

No padding, this looks better imo:

And here's the square version (35px × 35px each):