Scatter Plots and Extensions

The scatter plot is possibly the single most important statistical graphic. In this chapter we discuss the xyplot() function, which can be used to produce several variants of scatter plots, and splom(), which produces scatter-plot matrices. We also include a brief discussion of parallel coordinates plots, as produced by parallel(), which are related to scatter-plot matrices in terms of the kinds of data they are used to visualize, although not so much in the actual visual encoding.

A scatter plot graphs two variables directly against each other in a Cartesian coordinate system. It is a simple graphic in the sense that the data are directly encoded without being summarized in any way; often the aspects that the user needs to worry about most are graphical ones such as whether to join the points by a line, what colors to use, and so on. Depending on the purpose, scatter plots can also be enhanced in several ways. In this chapter, we go over some of the variants supported by panel.xyplot(), which is the default panel function for both xyplot() and splom() (under the alias panel.splom()).