hang on vs plug away

hang on

verb
  • To persevere. 

  • To hold, grasp, or grip. 

  • To pay close attention to, or regard with (possibly obsequious) admiration. 

  • To depend upon. 

  • To keep; to store something for someone. 

  • To continually believe in something; to have faith in. 

  • To wait a moment. 

plug away

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

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