Timbuktu vs back o' Bourke

Timbuktu

name

  • Any proverbially distant or remote place. 

  • A city in central Mali. 

back o' Bourke

name

  • An extremely remote place. 

adv

  • At or to an extremely remote place. 

How often have the words Timbuktu and back o' Bourke occurred in a corpus of books? (source: Google Ngram Viewer )