die out vs drop off

die out

verb
  • To cease gradually. 

  • To become extinct. 

drop off

verb
  • To lessen or reduce. 

  • To fall. 

  • To deliver; to deposit or leave; to allow passengers to alight. 

  • To fall asleep. 

How often have the words die out and drop off occurred in a corpus of books? (source: Google Ngram Viewer )