map to change dynamically as the mouse cursor is ed back and forth along the number line interestingly, the latter capability was provided in the developed at the Massachusetts Institu of Technology by Joseph Ferreira Wiggins (1990), Xmap used a "density dial" to highlight portions of a data set; Ferreira and Wiggins described the use of the density dial as follows: What makes the density dial a truly interactive visualiza tion tool is its ability to change the group of c that are highlighted in Ted as fast as we can move the mouse. We get an effect that like watching a video. We see a moving sequence of red-shaded polygons going from the least dense tract to the most dense tract, or vice versa. This speed and the sense of motion that comes with it enhances our ability to remember what we have just seen Unfortunately, xmap was not made generally available and so it is unlikely that you will have access to it. We will see, however, that the CommonGIS system pro- vides similar functionality Other major functions in ExploreMap include Regions, Tables, Graphs & Stats, and Utilities. Whereas Subset allows users to focus on a subset of data along the number line, Regions permits users to focus on a subregion of a mapped display of the data. For example, a politician interested in counties comprising an eco- nomic development region could use Regions to high- light enumeration units falling within that region. The Tables and Graphs & Stats functions focus on tabular graphical and statistical views of the data (as opposed to spatial views). For instance, Graphs & Stats can display a histogram and statistical parameters (Figure 22.4)