Different colours are Monthly Active Users, Weekly Active Users, Daily Active Users (MAU being highest).
May spike was due to https://news.ycombinator.com/item?id=27165366 making front page
Not sure there is a way to make Google Analytics public
We could pass to plausible analytics (for both forum and website?). Works very well and you can make dashboards public. I use it for nimib and related projects, for example here is the dashboard for scinim/getting-started: https://plausible.io/scinim.github.io%2Fgetting-started
Plan for clouds usage are reasonably priced and it is open source so with some dedicated effort you can put it on your own server.
As I mentioned in the website issue about removing Google analytics:
I have a paid subscription with plenty of views available (10k view monthly of which <500 currently used) and I would be happy to provide this as a service for nim websites as part of my sponsoring nim effort (I am not sure how many monthly views does the site gets, but if needed it would be ok also to bump the subscription to next tier which has 100K monthly views).
I guess website and forum are likely to reach higher than 100k monthly views, in that case maybe a subscription could be a good use of some of the available donation funds (in principle through events we could make public also telemetry for choosenim or other similar data).