Some consider this to be the first formal definition of a construct recognizable as a knowledge graph:
MS 514: “Existential Graphs” by Charles Sanders Peirce, circa 1882. It was published later within New Elements of Mathematics.
Are there other references about knowledge graphs prior to Gruber1993 ?
FWIW, I worked with Lenat, and his blackboard projects AM and Eurisko date to the mid-1970s, although he wasn’t explicit about using graphs (it was implied).