This would be useful to look at to have historic view that can be optionally enabled with log retention. I'm specifically looking at Connections tab providing you an overview of source and destination IP with bandwidth usage and their protocol.
ntop - no opkg, not as simple as pressing a button to setup
bandwidthd - no package available to download from opkg either
collectd - few addons are incompatible anymore such as collectd-mod-ipstatistics, collectd-mod-iptables
I assume you're going to do this on some x86, or other device with extendable storage, since the data you want to save would fill the available space pretty quickly.