keep on at vs persecute

keep on at

persecute

verb
  • To harass with importunity; to pursue with persistent solicitations; to annoy. 

  • To kill many of one species of animal, with the intent of removing them from human habitats. 

  • To pursue in a manner to do harm or cruelty to; especially, because of the victim's race, sexual identity, or adherence to a particular belief. 

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