Hello I first noticed that my system monitor related widgets are not displaying anything anymore save for Applications table which is still normal.
The widgets are there but it seems like they're not having any data input they can draw curves from. (line chart)
So I checked the system monitor as launched from Application menu and the Overview tab is blank save for its titles (Memory, Disk, CPU, etc) until Applications below which seems to work and update as usual. The other tabs reflect this situation.
I've recently upgraded my system. Maybe something went wrong there? How can I see what I uninstalled, maybe something was removed that should have really stayed.

Abolutely typical. Ksystemstats had been uninstalled. Sudo apt install ksystemstats. Thank you all for your help.