More interesting than the tracking of single terms is comparison. Look, for instance, at the relationship between phonetics and phonology (click to embiggen, as always):
Or take the rise of a basic new field, sociolinguistics, compared to a more traditional related area, dialectology:
Or take a particular principle/law, using neogrammarian and Neogrammarian because ngram is case sensitive, compared to lexical diffusion:
Even more fun are comparisons of particular theories or frameworks ... see for yourself.