Refers to Benjamin Harrison, senator from Indiana and 23rd President of the US. His name appears in a reference to the 1888 Republican National Convention (https://en.wikipedia.org/wiki/1888_Republican_National_Convention).
The graph displays the other subjects mentioned on the same pages as the subject "Harrison". If the same subject occurs on a page with "Harrison" more than once, it appears closer to "Harrison" on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.