hold on vs hold the line

hold on

verb
  • To persist. 

  • To remain loyal. 

  • To grasp or grip firmly. 

  • To keep; to store something for someone. 

  • Wait a short while. 

hold the line

verb
  • To firmly maintain one's viewpoint, principles, or situation; to refuse to change one's practices or plans. 

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