onanism vs self-gratification

onanism

noun

  • Masturbation. 

  • Ejaculating outside the vagina during intercourse; (the performing of) coitus interruptus. 

self-gratification

noun

  • Masturbation. 

  • Self-satisfaction. 

How often have the words onanism and self-gratification occurred in a corpus of books? (source: Google Ngram Viewer )