*Mousing over the above graph reveals that several series' have gaps. This is because 74 songs (72 Instrumental and 2 Incomprehensibly Foreign) have been omitted. Refer to Question 223 for definition of the Instrumental and Incomprehensibly Foreign categories.
NOTE: As of the April 20 2015 it is apparent that an increase in unique words used in songs have risen due to increased use of slang over the last decade. An analysis seeking to compensate for slang is desirable such that words like dem or dat map to them or that, respectively, and that both words, although distinct, would be considered the same. By mapping slang to traditional English in this way, also know as stemming, one is able to compare older music to modern music apples to apples. Since there is an obvious case for also tabulating slang words as unique words, in that such words are significant because they describe the evolution of musical expression, a comparison of both approaches will be provided here May 4th 2015