cool off vs peter out

cool off

verb
  • To cause to decrease in temperature, activity or temper. 

  • To decrease in temperature, activity, or temper. 

peter out

verb
  • Synonym of peter (“originally (mining), of a vein of ore: to be depleted of ore; now (generally), to diminish to nothing; to dwindle, to trail off”) 

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