drop off vs let up

drop off

verb
  • To lessen or reduce. 

  • To fall. 

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

  • To fall asleep. 

let up

verb
  • To cease or stop. 

  • To cease or stop (doing something intense). 

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