If you run Usage on Mobian (incl. in their Phosh flavour), you might have already seen the network tab, but for those on postmarketOS:
With the power of vnstat, Usage will show your network traffic per interface—including most importantly, your mobile data connection. You just need vnstat.
It is rudimentary, and recently I've been more interested in making the rest of the app more sleek and performant. It might interest you nonetheless. (And I'm always open to discuss and review improvements.)


