Argus-eyed vs on guard

Argus-eyed

adj

  • ever vigilant 

on guard

adj

  • Vigilant, alert. 

  • The starting position of a fencing bout; en garde. 

  • Transmitting on the aircraft emergency frequency. 

How often have the words Argus-eyed and on guard occurred in a corpus of books? (source: Google Ngram Viewer )