skill set vs string to one's bow

skill set

noun
  • A set of skills. 

  • The skills needed to accomplish a specified task or perform a given function. 

string to one's bow

noun
  • A skill, ability or resource. 

  • A lover, paramour or suitor. 

How often have the words skill set and string to one's bow occurred in a corpus of books? (source: Google Ngram Viewer )