I wanted to sum bandwith values for interfaces of 9 devices, which I was able to do by now, but I really want to just plot the sums, not every single interface needed for the sum. This is done in plain RRD by using CDEFs, but here there is not that option.
Yes... we're currently lacking a generic CDEF abstraction.
We have handled this particular senario by adding all of the graph items you intend to sum, first as an AREA, and subsequently as STACKs, but all with blank labels. RRDTOOL should hide these from the legend. We would then add an AREA that was a 'Sum of all items' (which invokes a CDEF), and use that as the aggreate. IIRC, we have it such that every time you use a 'Sum of all items' it will reset the list, allowing you to have multiple summed groups per graph. It doesn't work in all situations, but it does work for making an aggregate traffic graph that looks somewhat appealing.
Let us know how you make out.
I'm doing what you describe already. If you look at my example graph above I can display the sums the way I want. What I really didn't want to display is the lines(and areas whatever) from the single devices. That is noticeable on the several blue lines down on the graph.
Ok... I see what you mean. We were able to get away with it because we didn't show the individual links on our graphs, only the sum. CDEF support is on the TODO list, hopefully we'll have it done sooner rather than later.