I know that there is an option to script the router to monitor traffic (DPI, etc), I found it a little complicated to setup. Also, it requires an external software to actually monitor the traffic like ntop. I think it would be very useful for people to have a very basic data usage graph for metered internet. I know I'm not the only one that has a limited Fair Usage Policy from their provider. Although the average bandwidth like the current one that we can set is useful to see how the network/internet is being used and what times the traffic peaks are, a how much data is being used (both up and download).
I have been able to monitor the usage from Queues based on Mangle rules, but when I reboot the router, the counters reset to 0. If somehow that doesn't reset on a reboot alone is already helpful, plus it keeps the data on a per day, per week, per month, and per year basis.
Just a thought.