ax to grind vs rigor

ax to grind

noun
  • A grudge or grievance, with implications of confrontation. 

  • A proposition or point of view to advocate, especially motivated by self-interest. 

rigor

noun
  • A feeling of cold with shivering accompanied by a rise in body temperature. 

How often have the words ax to grind and rigor occurred in a corpus of books? (source: Google Ngram Viewer )