Good idea. If we were to roll our own analytics using Piwik, we could do this easily. Or at least, we could do it, i don’t know how easy it would be! Actually, Piwik supports this natively, so it’s quite trivial.
I think we’re bit busy to make such a report, but if you’re interested, maybe you’d like to do it yourself? Let me know and I can grant you access to our analytics account.