donut hole vs doughnut hole

donut hole

noun
  • The empty space in the center of a donut. 

  • A small ball-shaped pastry, made in the same manner as a donut, roughly the size of the hole in a donut. 

  • A section of water for which no one has fishing rights that is completely surrounded by regions where various parties hold fishing rights. 

  • An income bracket where one is without benefits, even though benefits are available in the next lower and next higher brackets. 

doughnut hole

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