This is common. The thing is, other structures may be and are often represented as a low level as graphs (a set of nodes and directed edges). For example, taxonomies are hierarchical trees, but they are commonly represented as graphs (for example, a taxonomy may be represented using the SKOS vocabulary for RDF graphs). A tree may be thought of as a directed graph where nodes have at most one outgoing edge ("parent"). A list may be thought of as a directed graph where nodes have at most one incoming edge and at most one outgoing edge. So, graph representation may be a "universal solvent" for all data structures of interest to you.
