Relational *key* people (sorry, "star" was bad choice of word, unless I
cited Kleene too:-) you know and cited two, Codd, Chen, and then
there's Date, Ulman, Wiederhold, etc. and I would not mind citing here
the people siting at the W3C working groups for XQuery, XPath, etc.
E-R maps better to graph-based representations than to relational ones.
It is not a matter of "can" but of "how well."
No I won't mention UML nor say the dirty word ("semi-structured":-)
There's lots of (good) graph-based models. RDF is (very) different from
XML and both are (very) different from Codasyl and my baby called
Mneson is (very, not so much, less) different from Codasyl, XML, RDF.
(I suggest Mneson be discuss in the thread "Implementing a graph
algebra" because that's it)