imprison vs pen in

imprison

verb
  • To put in or as if in prison; confine somebody against their will. 

pen in

verb
  • To cause to be stuck in a situation; to be forced into a situation by circumstances. 

  • To enclose (animals) in a pen. 

  • To schedule; fix a date for. 

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