At this point, lower tobacco leaves were pulled as they ripened, but the remaining tobacco plant was still cut at harvest. [Tilly, The Bright Tobacco Industry 1860-1929.]
The graph displays the other subjects mentioned on the same pages as the subject “tobacco cutting”. If the same subject occurs on a page with “tobacco cutting” more than once, it appears closer to “tobacco cutting” on the graph, and is colored in a darker shade. The closer a subject is to the center, the more "related" the subjects are.