Handsy -
Not sure it is possible to have just one datapoint on the graph to represent 2 interfaces for example, but you can stack them together to show that total utilization was 3Mbps.
If you go to the graphs definition section of the report and select the link under the name section of the the graph point(s) you have defined, you change the stacked section to true. This will continue to show your 2 (or more) of the interfaces on the same graph, but they will be stacked to show total usage.
Hope this helps,
--Rob