diminish vs slacken off

diminish

verb
  • To become less or smaller. 

  • To taper. 

  • To disappear gradually. 

  • To take away; to subtract. 

  • To make smaller. 

  • To lessen the authority or dignity of; to put down; to degrade; to abase; to weaken; to nerf (in gaming). 

slacken off

verb
  • To slacken or lessen; to become less intense. 

  • To do (something) less intensely. 

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