If you increase the date range to 12 months, you see that (with "likely human"), linux usage jumped from 3-4% to 5%-6% in the last 1.5 months. Something changed.
ChromeOS jumped down several points in the same time period. So maybe some update of ChromeOS got misconfigured?
Is my understanding of that graph right? if the periodic change is the weekends, that means the total share percentage of requests from Windows and macOS users drops, while Linux increases in share percentage. Linux traffic is stable regardless of weekday/weekend cycles compared to Windows and macOS? Otherwise if there was equal drop in Linux, then the share percentage would be a smoother line. Is that right?
You can't tell from this graph alone the extent to which the change in share during weekends is due to decreased use of the more popular operating systems or due to increased use of Linux.
You have to buy a Chromebook to run ChromeOS, right? You can install Linux on your PC as dual boot. The investment is far smaller. However you have to buy a Mac to run MacOS so maybe that means that a Chromebook is not a very compelling machine to buy, or has a limited use case.
I don't believe this for a moment. I have seen many more Chromebook users than desktop Linux users. Not individuals, but a lot of schools use Chromebooks where I am. They are probably not well-tallied in the results, because they are often used to do some work in Google Docs/Sheets, plus some education-specific sites and less browsing on the general web.
I guess my point is that it is nearly impossible to get reliable counts from user agent statistics, etc. (lots of crawlers, that identify as a regular desktop as well etc.). The best way would probably to do real-world surveys among random population samples. But that costs money/time.
If you increase the date range to 12 months you can see that ChromeOS is several times larger than Linux but recently there must have been an update so ChromeOS is now incorrectly detected as Linux.
6%-14% (throughout the day, for some reason it's most popular late at night, Steam Box and other gamers, maybe?) is still the largest it's ever been, though, and it's presumably not getting smaller.
It's about the same as Firefox share, which is getting smaller.
ChromeOS jumped down several points in the same time period. So maybe some update of ChromeOS got misconfigured?