ex cathedra vs mandatory

ex cathedra

prep
  • With authority based on one's rank or office. 

mandatory

adj
  • Of, being or relating to a mandate. 

  • Obligatory; required or commanded by authority. 

noun
  • A sign or line that require the path of the disc to be above, below or to one side of it. 

How often have the words ex cathedra and mandatory occurred in a corpus of books? (source: Google Ngram Viewer )