The best visualizations are not static images to be printed in books, but fluid, dynamic artifacts that respond to the need for a different view or for more detailed information. In some cases, the visualization can be an interface to a simulation of a complex system; the visualization, combined with the simulation, can create a powerful cognitive augmentation. An emerging view of human-computer interaction considers the human and the computer together as a problem-solving system (Zhang, 1997). In such a model, the data visualization becomes part of the interface between the human and machine system components. The visualization is a two-way interface, although highly asymmetric, with far higher bandwidth communication from the machine to the human than in the other direction. Because of this asymmetry in data rates, cognitive support systems must be constructed that are semiautomatic, with only occasional nudges from users steering them in a desired direction. The high-bandwidth visualization channel is then used to deliver the results of modeling exercises and database searches.

At the interface, the distinction between input and output becomes blurred. We are used to regarding a display screen as a passive output device and a mouse as an input device. This is not the way it is in the real world, where many things work both ways. A sheet of paper or a piece of clay can both record ideas (input) and display them (output). The coupling of input and output can also be achieved in interactive visualization. Each visual object in...