AFL vs American football

AFL

noun
  • The game of Australian rules football 

American football

noun
  • A game similar to rugby football played on a field of 100 yards long and 53 1/3 yards wide (with two 10 yard-long endzones) in which two teams of 11 players attempt to get an ovoid ball into each other's territory. 

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