"A person who is in charge of work done in a particular department, office, etc., or who is responsible for keeping a building or place in good condition." (CD)
The graph displays the other subjects mentioned on the same pages as the subject "Superintendents". If the same subject occurs on a page with "Superintendents" more than once, it appears closer to "Superintendents" on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.