Sum metrics from Influxdb

Hi.
I am currently trying to make a power graph of our PDU with grafana. I collect the data with CollectD and store them in our influxdb database.
I can show each phase as a graph, but i cannot figure out how to make a “total” graph which sums the values of the 3 phases.
Is that possible with influxdb (1.6.2) and grafana (5.2.1)

Best Regards,
Mads.