keep going vs plug away

keep going

verb
  • To continue functioning. 

  • To proceed; to make progress, physically or figuratively. 

  • To see that (a person) is continually supplied with something, or that they can progress or carry on. 

plug away

verb
  • To persist or continue, as with an effort. 

How often have the words keep going and plug away occurred in a corpus of books? (source: Google Ngram Viewer )