The graph displays the other subjects mentioned on the same pages as the subject "CHAS. G. BOELHAUWE,". If the same subject occurs on a page with "CHAS. G. BOELHAUWE," more than once, it appears closer to "CHAS. G. BOELHAUWE," on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.