ne plus ultra vs the zenith

ne plus ultra

noun
  • A prohibition against proceeding further; an insuperable obstacle or limiting condition. 

  • The highest, ultimate point of achievement which can be reached; perfection. 

  • The perfect or most extreme example of its kind; the ultimate. 

the zenith

How often have the words ne plus ultra and the zenith occurred in a corpus of books? (source: Google Ngram Viewer )