Another link that I need to thank Amy for finding:
Clavilux 2000 [jonasheuer.de] is a subtle music visualization installation that represents the playing of sounds by way of a simultaneous animation that can be interpreted.
For every note played on the keyboard, a stripe appears of which the dimensions, position and color correspond to the way the particular key was stroke. The length and vertical position of stripe is mapped unto the velocity, while the stripe's width reflects the length of each note. By mapping the color wheel on the circle of 5ths, the colors give the viewer (and listener) an impression of the harmonic relations. Notes belonging to one specific tonality correspond to colors from one specific area of the color wheel. Therefore each key has its own color scheme and "wrong" notes stand out in contrasting colors. The more different tonalities a music piece has, the more colorful the resulting visualization will be.
As all the stripes do not disappear, the resulting representation is able to convey insights about the composition as well as the specific performance: Which notes were played the most? Which were the loudest notes? Which range of the keys was played mostly? How harmonically constant was the music?