cling to vs retain

cling to

verb
  • To remain by (someone's) side; to refuse to leave the company of. 

  • To have an intense emotional dependency on (someone). 

  • Used other than figuratively or idiomatically: see cling, to. 

retain

verb
  • To keep in possession or use. 

  • To employ by paying a retainer. 

  • To hold back (a pupil) instead of allowing them to advance to the next class or year. 

  • To keep in one's pay or service. 

  • To hold secure. 

How often have the words cling to and retain occurred in a corpus of books? (source: Google Ngram Viewer )