A graph is a set of objects, some pairs of which are related. Graphs are the closest thing CS has to a universal data type.
Graph databases are niche but well-studied.
Fulcro: Full-stack CLJ(S) application framework focused on graph data
Pathom: CLJ(S) library for writing graph query parsers
Lots of Semantic Web stuff focuses on graphs. A set of "semantic triples" in the RDF sense defines a graph.
XTDB: Graph database with bitemporal indexing