ax to grind vs fad

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. 

fad

noun
  • A phenomenon that becomes popular for a very short time. 

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