Matt Jensen of NewsBlip has pointed me to work he and others are doing on historical visualization. See, for example, the technical report on Semantic Timelines that he wrote. Historical Visualizations, by David Staley in the JAHC: Journal of the Association for History and Computing (vol. III, no. 3, Nov. 2000) surveys the ways visualization is used in history from concepts to timelines.
Another similar project is the Temporal Modelling project led by Johanna Drucker. She and Bethany Nowviskie have been working on visual forms of knowledge production. See their Prototype Designs for a temporal vocabulary which I blogged earlier at 3D Timelines.