  The Information Artifact Ontology is an ontology in the domain of information
entities. Core to the definition of what it is to be an information entity is
the claim that an information entity must be `about' something, which is
encoded in an axiom expressing that all information entities are about some
entity. This axiom comes into conflict with ontological realism, since many
information entities seem to be about non-existing entities, such as
hypothetical molecules. We discuss this problem in the context of diagrams of
molecules, a kind of information entity pervasively used throughout
computational chemistry. We then propose a solution that recognizes that
information entities such as diagrams are expressions of diagrammatic
languages. In so doing, we not only address the problem of classifying diagrams
that seem to be about non-existing entities but also allow a more sophisticated
categorisation of information entities.
