Interest in visualization-based user interfaces has blossomed in the past few years, with systems developed for application areas from hypertext information to geology, molecular biology, file system structure, and animal behavior patterns. Large visualization systems contain modules that users can hook together into a data-flow network to create visualizations. These systems offer many advantages for rapidly building applications. The success of these systems attests to the utility of modular, easy-to-use, extensible tools for visualization tasks. Examples of such systems include ConMan [9], AVS [29, 32], IRIS Explorer [34], IBM Data Explorer [33], and Visualization Toolkit (VTK) [25, 26].