Nana vs Nanna

Nana

name
  • One's grandmother. 

  • A governess. 

  • A Japanese female given name from Japanese 

Nanna

name
  • A goddess, the wife of Balder. 

  • A surname from Italian. 

  • The god of the moon in Sumerian mythology. 

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