strength vs weak spot

strength

noun
  • The intensity of a force or power; potency. 

  • The quality or degree of being strong. 

  • The strongest part of something; that on which confidence or reliance is based. 

  • A positive attribute. 

weak spot

noun
  • A location where the defenses are weak or the vulnerability is great. 

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