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MEDIATION


Triadic information, according to Peirce, describes the relation between two things mediated by a third. There are many ways in which mediation can be used in statistical graphics. Generally, mediation involves comparison of plots displayed simultaneously on the screen or page, or over time on the screen as in dynamic graphics.

In a statistical context, the most common form of mediation is by the value of some categorical variate. For example, a categorical variate Z mediates the relationship between the Y and Xvariates of a scatterplot when point symbols are coloured according to their Z values. We could also show the relationship between two variates mediated by a third by laying out a grid of pointclouds, one for each distinct value of Z. A mechanism for producing these kinds of plots is the so-called batch-plot, a kind of grid-plot whose interior view is a batch-layout with associated axes and labels in the margins.

Much of interactive statistical graphics involves some form of mediation. Brushing ``linked'' plots is perhaps the best known example. In the Quail system, there are two different implementations of linking. The first (see Hurley and Oldford, 1991) is the most efficient but also the most restrictive. At any time, a single view object can be a component of more than one compound view. The scatterplot matrix makes use of this feature: there is one point symbol for each case and this point symbol is a component of all the point clouds in the matrix. Since there is just one point symbol for the case, the point symbol has just a single drawing style and the change of highlight status via the brush in one location is automatically reflected in the other point symbol locations.

The second linking implementation (Hurley, 1993, 1997) is far more flexible. Basically, any two simple views can be linked to each other, with the result that a change of drawing style in one view is reflected in a change of drawing style in the second view. In the default mode of operation, two simple views are linked to each other if they display ``the same'' data, but, more generally, the user controls which views should be linked by choosing or specifying a function which, given a pair of views, decides whether or not they should be linked.


next up previous
Next: CONCLUDING REMARKS Up: No Title Previous: Grid Layout

2000-01-28