Unions are not properly represented

[dgarijo]

If I have that a property has an union of two classes as domain, the diagram is not very helpful. Ideally it would be great to show something like this: http://vocab.linkeddata.es/vgo/images/game-product-model.svg. What is shown at the moment is very confusing (like an attempt of an rdf list, but with a huge NULL node).

[idafensp]

This is going to take a while to implement, but is a great idea. Ar2dtool so far just translates triples into graph elements, which is not straightforward in this case.