It's worth noting that Virtuoso (produced by my employer, available in free Open Source and paid Commercial variants, http://virtuoso.openlinksw.com/features-comparison-matrix/) is a hybrid Relational/Graph/XML/FreeText storage and query engine, which natively supports SQL, SPARQL, XPath, XQuery, and many other open standards. It might satisfy the OP's needs on its own.

Virtuoso's support for open standards makes it easy to use it as a complete solution covering all the bases, or, as in @philjohn's case, to plug-and-play with best-in-breed solutions along any axis where our implementation proves not to serve your needs for any reason. (We do want to know how and why we don't measure up, so we can improve that aspect!)