We discuss an architecture and functionality of a multiagent system supporting data integration based on faceted query evaluation in a system of ontology-enhanced graph databases. We use an ontology (a subset of OWL 2 RL profile) to support a user in interactive query formulation. A set of server agents cooperate one with another in query answering using the ontological knowledge for query rewriting and for deciding about query propagations. We propose some algorithms ensuring tractable complexity of query rewriting and query evaluation.