Hi John,
I have come across with an open source solution- Cyberoam iView, which fits into your requirements.
Cyberoam iView provides centralized and consolidated logging and reporting for multiple devices/solutions as firewalls, proxies, billing solutions and full fledged UTMs.
This software provides comprehensive graphical reports for all the network activities. You can even drill down the reports up to multiple levels to get in-depth visibility of the overall network.
I would urge to give a try -
http://sourceforge.net/projects/cyberoam-iview/
Regards
Anima