LuCI doesn’t have pretty graphs of bandwidth usage by device by itself. Sorry. Majordomo can give you bandwidth usage totals for your devices, abet in text form, with a lot of detail.
Alternatives, you can install vnstat and associated packages to get some more charts in LuCI.
I’ve also had success in installing YAMon, which might be what you’re looking for. It required hacking up some configuration files though but it does appear to work.