pigskin vs rugby union

pigskin

noun
  • A football. 

  • Leather made from the skin of a pig. 

  • The skin of a pig. 

rugby union

noun
  • A form of rugby football, historically amateur, having 15 players per side; points are scored for a try, penalty, drop kick or conversion. 

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