See Robert Brough Smyth, Aborigines of Victoria, vol. 1, pp. 482-483
The graph displays the other subjects mentioned on the same pages as the subject “policeman Paddy”. If the same subject occurs on a page with “policeman Paddy” more than once, it appears closer to “policeman Paddy” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.