In cooperation with Martin Voigt from the chair of Multimedia Technology we recently published an (alpha) version of the Visualisation Ontology (VISO). Look at the VISO blog for details.
Technical Report on VISO
You can find documentation on the process of creating VISO and possible use cases in a technical report that we published: Towards a Unifying Visualization Ontology . [BibTeX]
VISO Modules
The following modules are online: