Mac XML Schema <-> UML tools. Which is best?

I am needing a tool that can slurp in an XML Schema (W3C format but if it does RNG too that would be killer as I prefer it) and show a UML Class View Diagram of the data model. It must run natively on Mac OS X. I would like a tool that recognizes that UML 2.0 can express a superset of an XML model (contain additional constraints), yet be able to convert from the UML to the XML expression of a given data model and preferably generate any warnings of lost constraints/semantics/cardinality rules etc. I do not want to use XMI if I can help it.

Any advice or recommendations would be greatly appreciated. Open source preferred!

Duane