pig pile vs pig-pile

pig pile

noun
  • A disorderly pile of people formed by jumping upon a victim. 

verb
  • To jump into such a pile. 

  • To cause a group of people to lie in a pile upon another, originally as a punishment to the victim on the bottom. 

  • To act similarly with regard to residential density: to live or cause to live in high-density settlements. 

pig-pile

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