Users need to increase to more than the maximum of 15 charts of On-Demand multi-metric views.
Don't see any corresponding attribute in the Performance Center Global Attribute page.
Increasing above 15 in the report setting generates an error.
Re-tested that with same result:
It might help when you increase the "OnDemand.MaxItemsSelected.Limit" Parameter from 15 to a higher value... not tested so far, did anyone test it ?