ProfessorI realize the original request in this thread is from 5 years ago, but since it popped up again, perhaps still useful to respond :).
Since this is a known set of devices, the problem originally stated can be solved with a widget that has each interface as an element (hidden optionally), then a CDP with the sum of all interfaces as the displayed element. Admittedly, the widget builder should be a smarter about selecting datapoints via query so you could automate this task based on interface attributes, but you could still create the widget and make a point to update it when the network expands. In theory, the smart-widget maintenance bit could be done via the API. If you need it emailed, setup a dashboard report to handle that piece (this may not have been possible 5 years ago).
If you are literally looking for total GB/day for one or more interfaces, then that is not going to happen with a widget or alerts (both of which should be possible -- I am getting increasingly spoiled by how flexible Grafana is :)). That could be accomplished by building the widget, then use the API to download the widget's data and transform as needed.
That transformation could also be done within a Groovy datapoint using your Nth copy of the same API code (maybe the same, or maybe you edited 10 of them to fix a bug and the other 5 still have the bug -- what common programming feature could possibly help with that?).
