light a fire under vs stir up

light a fire under

verb
  • To motivate or encourage (someone) to start sooner or move faster. 

stir up

verb
  • To arouse or excite (passion or action). 

  • To move or disturb slightly; to make turbid. 

  • To mix (ingredients) by stirring. 

How often have the words light a fire under and stir up occurred in a corpus of books? (source: Google Ngram Viewer )