Flickr Flow is an experiment whose materials are color and time. It has been created by Fernanda Viergas & Martin Wattenberg.
Using an algorithm developed for the WIRED Anniversary visualization, the software calculated the relative proportions of different colors seen in photos taken in each month of the year, and plotted them on a wheel. The image below is an early sketch from the piece. Summer is at the top, with time proceeding clockwise.

This technique has been used in a Boston Magazine piece, Flickr of Hope, that appeared in March 2009.

http://hint.fm/projects/flickr/
http://hint.fm/projects/wired2008/