drag on vs keep on

drag on

verb
  • to last too long 

keep on

verb
  • To persist or continue. 

  • To cause or allow to remain in the on or open position. 

  • To cause or allow to remain in an existing position. 

  • To persist in talking about a subject to the annoyance of the listener. 

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