placate vs pour oil on troubled waters

placate

verb
  • To calm; to bring peace to; to influence someone who was furious to the point that they become content or at least no longer irate. 

pour oil on troubled waters

verb
  • To calm, appease or quieten a person or situation. 

How often have the words placate and pour oil on troubled waters occurred in a corpus of books? (source: Google Ngram Viewer )