guqin vs koto

guqin

noun
  • A plucked zither-like stringed instrument (chordophone), traditionally featuring seven unfretted strings, originating in ancient China. 

koto

noun
  • A Japanese stringed instrument having numerous strings, usually seven or thirteen, that are stretched over a convex wooden sounding board and are plucked with three plectra, worn on the thumb, index finger, and middle finger of one hand. 

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