dwindle vs slacken off

dwindle

verb
  • To lessen; to bring low. 

  • To decrease, shrink, diminish, reduce in size or intensity. 

  • To break up or disperse. 

  • To fall away in quality; degenerate, sink. 

slacken off

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

  • To do (something) less intensely. 

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