With Cacti 1.x, you do this with aggregate graphs.
First, you create single graphs for every CPU found on your device (Devices->(your device)->create graphs for this device->create all graphs for the "Data Query [SNMP - Get Processor Information]" table)
Then you go to Management->Graphs and filter the CPU graphs of your machine you just created. Check all you want in your aggregate graph.
Then click on the "Choose an action" drop down list on the bottom and choose "Create an aggregate graph". This will create a single graph with all single data sources merged (or aggregated). No tedious manual assignments of several data sources into a single graph.
I don't know if you can delete all the single graphs afterwards (while keeping their data sources), didn't try that yet. I assume it is required to keep them, because the graph id's appear as items of the aggregate, not the data source id's or graph template id's.
Thank you so much, that did it.
I just need to play around with this, but this is great.