Mum vs mammy

Mum

name
  • One’s mother. 

mammy

noun
  • mamma; mother 

  • In the southern United States, a black nanny employed to look after white children; or in the antebellum South, a female slave who was close to the household and looked after the children. 

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