follow up vs hold on

follow up

verb
  • To take further actions remaining after an event; to continue, revisit, or persist; especially, to maintain communication or verify. 

hold on

verb
  • To persist. 

  • To remain loyal. 

  • To grasp or grip firmly. 

  • To keep; to store something for someone. 

  • Wait a short while. 

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