atabeg vs viceroy

atabeg

noun
  • A high medieval Turkish feudal title, originally charged with the caretaking and mentoring of the realm's crown prince. 

viceroy

noun
  • One who governs a country, province, or colony as the representative of a monarch. 

  • An orange and black North American butterfly (Limenitis archippus), so named because it is similar to, but smaller than, the monarch butterfly. 

  • A zongdu. 

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