free rein vs power of attorney

free rein

noun
  • The absence of constraints; freedom to make decisions. 

  • Loose rein(s), as of a horse. 

power of attorney

noun
  • The legal authorisation of one person to act as the agent of another. 

  • A legal document allowing one person to act as the agent of another. 

How often have the words free rein and power of attorney occurred in a corpus of books? (source: Google Ngram Viewer )