This probably refers to Billy Tulaba, otherwise known as TulabaTulaba, also known as Billy McLeod was a key Gippsland informant to Howitt. His name is variously spelt Toolabar, Tulabar, Talaba etc.
The graph displays the other subjects mentioned on the same pages as the subject “Billy”. If the same subject occurs on a page with “Billy” more than once, it appears closer to “Billy” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.