Multivariate descriptive displays or plots are designed to reveal the relationship among several variables simulataneously..  As was the case when examining relationships among pairs of variables, there are several basic characteristics of the relationship among sets of variables that are of interest.  These include:

• the forms of the relationships

• the strength of the relationships, and 

• the dependence of the relationships on external (usually to the pairs of variables being examined) circumstances.

Multivariate plot examples

• enhanced 2-D scatter diagrams

• 3-D scatter diagrams

• contour, level, and surface plots

• high-dimensional data plots

Trellis graphics

Many data sets include a mixture of both "continuous" (ordinal-, interval- or ratio-scale variables) and "discrete" (nominal-scale variables).  Often, the issue might arise of how a particular relationship between variables might differ among groups.  Information of that nature can be gained using conditioning plots (or coplots).  Such plots are part of a general scheme of visual data analysis, known as Trellis Graphics that has been created by the developers of the S language.  Trellis Graphics are implemented in R using the package Lattice.

• coplots (conditioning plots)

• Lattice plots

• spplot() Lattice-type maps

The main documentation for Trellis graphics includes:

• Trellis Graphics User Manual, and

• A Tour of Trellis Graphics

two .pdf documents published by the developers of the S language and Trellis Graphics, Lucent Technology.

Readings:

Kuhnert & Venebles (An Introduction...):  p. 86-96, 179-201;  Rossiter (Introduction ... ITC):  sections 5.4-5.6