ax to grind vs motivation

ax to grind

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

  • A grudge or grievance, with implications of confrontation. 

motivation

noun
  • Something which motivates. 

  • A research rating that measures how the rational and emotional elements of a commercial affect consumer intention to consider, visit, or buy something. 

  • The action of motivating. 

  • Willingness of action especially in behavior. 

  • An incentive or reason for doing something. 

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