impugn vs pick apart

impugn

verb
  • To verbally assault, especially to argue against an opinion, motive, or action; to question the truth or validity of. 

pick apart

verb
  • To criticise (especially small details). 

  • Used other than figuratively or idiomatically: see pick, apart. 

  • To overcome by skilled execution. 

  • To review or analyse in great detail 

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