perseverate vs plug away

perseverate

verb
  • To persist in doing something; to continue to repeat an action after the original stimulus has ended. 

  • To cause the perseveration of (a given reflex or response). 

plug away

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

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