dirty protest vs human chain

dirty protest

noun
  • A form of protest where a prisoner refuses to use the toilet and instead defecates and urinates within his or her cell. 

human chain

noun
  • A form of demonstration in which people link their arms as a show of political solidarity. 

  • A bucket brigade. 

How often have the words dirty protest and human chain occurred in a corpus of books? (source: Google Ngram Viewer )