hold onto vs keep back

hold onto

verb
  • To manage to keep something. 

  • Used other than figuratively or idiomatically: see hold, onto. 

keep back

verb
  • To stay back; to refrain from approaching. 

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

  • To restrict or restrain. 

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

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