Likely, Mr Thomas Petrie, a goldminer, grazier and Indigenous welfare officer. See: http://oa.anu.edu.au/obituary/petrie-thomas-tom-4395
The graph displays the other subjects mentioned on the same pages as the subject “Thos Petrie”. If the same subject occurs on a page with “Thos Petrie” more than once, it appears closer to “Thos Petrie” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.