hard rock vs ragtime

hard rock

noun
  • A rock music genre marked by a heavy regular beat, high amplification, and usually frenzied performances. 

ragtime

noun
  • A musical form having a rhythm characterized by strong syncopation in the melody with a regularly accented accompaniment. 

  • A piece of music in this style. 

How often have the words hard rock and ragtime occurred in a corpus of books? (source: Google Ngram Viewer )