niggle vs nitpick

niggle

verb
  • To dwell too much on minor points or on trifling details. 

  • To use, spend, or do in a petty or trifling manner. 

  • To fidget, fiddle, be restless. 

noun
  • A minor complaint or problem. 

nitpick

verb
  • To correct insignificant mistakes or find fault in unimportant details. 

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