keep back vs postpone

keep back

verb
  • To restrict or restrain. 

  • To hold back; to refuse to give or share. 

  • To prevent (a pupil) from advancing in a course. 

  • To stay back; to refrain from approaching. 

postpone

verb
  • To delay or put off an event, appointment, etc. 

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