This is pretty easy to do in Performance Analytics (even if a bit obscure), but there may not be a good way to do it in basic reporting.
With Performance Analytics, you'll have your Indicator with Number of Closed Incidents (or Number of Resolved Incidents, depending) with the Assignment Group breakdown as per normal (this should be out of the box with the Performance Analytics license ... if not, it'll be part of the free content pack)
As Jeff mentioned below, you'll want to make a new Formula indicator for it, and it'll be SOMETHING like this:
{{Number of Closed Incidents}} / [[Number of Closed Incidents]] * 100
(I may have those backwards, and make sure the name is correct)
but one set of braces means "Include breakdowns" and the other means "Don't include breakdowns". If you use the builder, it writes it for you appropriately depending on whether you check "Include breakdowns" or not.
Then just include whatever breakdowns you want it to follow. The numerator will follow the breakdowns and the denominator will maintain the total.
It DOES mean that the initial, un-broken down number will just be 100% which isn't terribly helpful, but choosing any breakdown element will give you the breakdown element's percent of the whole.
Hopefully this helps!
P.S. If you don't own Performance Analytics either by the original SKU or as part of an ITSM Pro or Enterprise SKU, SOME PA indicators and dashboards, particularly for Incident are still included out of the box at no cost. I don't recall if we have one exactly like this already as part of the free package, but it's worth a peek, even if you don't have the full license.
