Keystone vs Tombstone

Keystone

name

  • A town in South Dakota. 

  • A city and town in Iowa. 

  • A census-designated place in Colorado. 

  • A ghost town in North Dakota. 

Tombstone

name

  • A city in southeastern Arizona. 

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