sheepskin vs welcome mat

sheepskin

noun
  • A diploma. 

  • The tanned skin of a sheep with the fleece left on, especially when used for clothing, rugs, etc. 

  • The skin of a sheep, especially when used to make parchment or in bookbinding. 

welcome mat

noun
  • Any sign or indication of welcome. 

  • A doormat with a welcoming message on it. 

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