grist for the mill vs serendipity

grist for the mill

noun
  • Something that is useful or that creates a favorable opportunity. 

serendipity

noun
  • The occurrence and development of events by chance in a happy or beneficial way. 

  • An unsought, unintended, and/or unexpected, but fortunate, discovery and/or learning experience that happens by accident. 

  • A combination of events which have come together by chance to make a surprisingly good or wonderful outcome. 

How often have the words grist for the mill and serendipity occurred in a corpus of books? (source: Google Ngram Viewer )