lessen vs step up

lessen

conj
  • unless. 

verb
  • To make less; to diminish; to reduce. 

  • To become less. 

step up

verb
  • To increase, augment, or advance, especially by one or more steps. 

  • To assume or take responsibility. 

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