Achilles heel vs soft spot

Achilles heel

noun

  • A vulnerability in an otherwise strong situation. 

  • The Achilles tendon, the tendo Achillis. 

soft spot

noun

  • A point of vulnerability in a defence. 

  • A sentimental fondness or affection. 

  • A fontanelle. 

How often have the words Achilles heel and soft spot occurred in a corpus of books? (source: Google Ngram Viewer )