Refers to Russell Alger, senator, governor of Michigan and Secretary of War under McKinley (https://en.wikipedia.org/wiki/Russell_A._Alger). Terrell refers to the fact that he was a presidential nominee during 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 "Alger". If the same subject occurs on a page with "Alger" more than once, it appears closer to "Alger" on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.