plummet vs slacken off

plummet

verb
  • To drop swiftly, in a direct manner; to fall quickly. 

noun
  • A violent or dramatic fall. 

  • A decline; a fall; a drop. 

slacken off

verb
  • To do (something) less intensely. 

  • To slacken or lessen; to become less intense. 

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