A supernova is a star saying "The End". The H-R diagram shows they different types of stars by spectral class, color, etc. It was never intended to show the ending of stars. On most H-R diagrams, you will find at the top, or occasionally top right, a place for "Supergiants". Some of those stars will eventually become supernovas. To be absolutely clear: A supernova isn't a type of star - it is the "end" of a type of star.
