Here’s an actual answer, a system monitor with historical data: https://beszel.dev/

It’s a webUI but that shouldn’t really matter vs an app with its own GUI.

If you don’t mind a web UI, Netdata is great. It collects a bunch of metrics once per second and can retain them for a long period of time. The web UI is pretty good. Their Github readme links to some example servers so you can try it out first. Just click the link to use it without an account (that’s optional).

It’s mainly designed for servers, but there’s no reason you couldn’t run it on a client system. They’re focusing a lot on AI/ML-based anomaly detection as well as their cloud offering at the moment, but you don’t have to use either and can just stick to the open-source agent.

btop? it’s pretty customizable, if a bit too flashy (by default) to my liking. https://github.com/aristocratos/btop - should be available on repositories for most distros.

You may have a bit of a hard time finding something that’s completely FLOSS that’s not on the older side (the sar visualizer being a Java desktop application being a consequence of that age). There are various ways to dump resource usage into a time series database like Prometheus (Apache2), InfluxDB (Apache2/MIT), or VictoriaMetrics (Apache2) and then visualize it with a frontend (Grafana, APGL). The database is going to be the tricky part. All of the time series DBs I’m aware of are permissively licensed. Grafana may be a good fit for you, however. It’s written in Go so it’s relatively light, although it obviously requires a browser to interact with.

Atop, btop, htop, top.

If that’s not good enough sar.

If that’s not good enough, set up cacti.

When you realize none of that stuff is actually helping you, journalctl and grep.