bother vs keep on at

bother

verb
  • To do something which is of negligible inconvenience. 

  • To feel care or anxiety; to make or take trouble; to be troublesome. 

  • To annoy, to disturb, to irritate. 

noun
  • Fuss, ado. 

  • Trouble, inconvenience. 

intj
  • A mild expression of annoyance. 

keep on at

How often have the words bother and keep on at occurred in a corpus of books? (source: Google Ngram Viewer )