I can't say I know exactly what a Data Flow Diagram is, but have you checked out OmniGraffle? It's not free, but they do have a free trial, and it's not too expensive for the full version. Just released a new version too.
As well, the OmniGraffle Pro version has compatibility with Vizio, which is popular on Windows. I've only used an early version but it's quite slick to use.