It would be very helpful if there were a way to programmatically make a font or a line on a diagram (e.g. the bounding box of a class symbol, an association, trenary role, dependency etc.) invisible (not just white) in the same way as a symbol's background/fill to "hollow". Using scripting, this would then make it posible to implement some kind of "layering" of diagrams as known from CAD systems, Visio, or even Sparx Enterprise Architect.
Upon request, I can provide a quite sophisticated prototype implementation of such a layering mechanism with the limitation that the background of symbols must be empty or white (fonts and lines are changed to "white" to make them invisible).