RTG uses MySQL for it's backend, so you can basically setup queries however you like and you can use RTGPOLL to graph multiple interfaces as well.
It's a super good tool and I think there is a group working on RTG2 at googlecode (I think).
Another RTG user! I didn't know many of us existed! RTG is a great tool. It's design (perl and PHP and MySQL) lends itself to being modified at will; integration with tools like PHP NetworkWeathermap is very straightforward (http://pastebin.com/9RiZx4A8), and the MySQL backend makes it super flexible. There's no aggregation of data, unless you hack it in yourself with some fancy queries. RTG's data is ideal for doing MySQL partitioning, and there are some indexes that need to be added. But when you get those things in place, it becomes fast and powerful - and it's easy to drop out old data without a lengthy query (just drop the partition). The fact that each SNMP device gets its own table is also a big performance win over the more popular tools. The web interface allows for interface aggregation, and the code for doing that could probably be reverse engineered easily enough for other reporting mechanisms as well. Nathan Eisenberg