bad luck vs tragedy

bad luck

noun
  • An undesirable turn of events (such as an accident); misfortune. 

tragedy

noun
  • A disastrous event, especially one involving great loss of life or injury. 

  • A drama or similar work, in which the main character is brought to ruin or otherwise suffers the extreme consequences of some tragic flaw or weakness of character. 

  • The genre of such works, and the art of producing them. 

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