SuperMetric for Aggregate Multiple Clusters CPU Demand %

Hi all-- the subject about sums it up.  I'm looking for a supermetric to aggregate multiple clusters %CPU Demand metrics into a single metric for my Custom Group.

So I created a Custom Group that contains a few dozen clusters.  I'd like to apply a supermetric on that custom group that combines all the individual demand % metrics into a single trend line.

Ideas?

Thanks!