I used to think that the open source xs3p schema documentation generator stylesheet from the now-defunct http://titanium.dstc.edu.au/ was the secret sauce behind the remarkably similar graphical XML schema representations of both <oXygen/> and XML Spy. I was wrong: a modified version of xs3p is bundled with <oXygen/> and is used when generating printed documentation, and xs3p may still be included in XML Spy (though it’s unlikely since its currently not listed on their third-party licenses page), but even in its titanium days, it didn’t do any graphical representations of a schema.
Does anybody know of an open source toolkit that can produce that sort of graphical representation?