singer vs vocalist

singer

noun
  • A person who sings, often professionally. 

  • A machine for singeing cloth. 

  • A person who, or device which, singes. 

  • dance figure with a fixed structure, sung by a caller, or a piece of music with that structure. 

vocalist

noun
  • A singer; a person who likes to sing. 

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