persist vs soldier on

persist

verb
  • To continue to exist. 

  • To go on stubbornly or resolutely. 

  • To repeat an utterance. 

  • To cause to persist; make permanent. 

soldier on

verb
  • To continue or persist, despite adversity or difficulty. 

How often have the words persist and soldier on occurred in a corpus of books? (source: Google Ngram Viewer )