Billy vs Rob

Billy

name

  • A diminutive of the male given name William. 

  • The B-25 twin-engine bomber aircraft used during World War Two, commonly called the "B-25 Mitchell" in honor of U.S. Army General William "Billy" Mitchell. 

Rob

name

  • A diminutive of the male given name Robert. 

  • A surname transferred from the given name, derived from Robert. 

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