Sounds like exactly what you want.
Change two or more interfaces descriptions to start with the same e.g testing: 123, then go to the URL /iftype/type=testing
(Note the semi-colon is important on the port description)
This will give you a page with aggregate graph for both interfaces, and each interface individually.
I can get you some examples tomorrow if you want,
Or create a bill with both interfaces added, Or, check out Single graph, Multiple interfaces