Michael and others,

Here is my complete ancova example

http://astro.ocis.temple.edu/~rmh/HH/hotdog.pdf


This example, especially in Figure 6, places them in a context of a
Cartesian
product of models with the intercept having two levels and slope having
three levels.

It is based on the ancova chapter from my book

\item Heiberger, Richard M., and Burt Holland (2004).
 {\it Statistical Analysis and Data Display: An Intermediate Course
      with Examples in \Splus, \R, and \SAS},
    Springer--Verlag, New York.
\url{http://springeronline.com/0-387-40270-5}.

and in essentially this form was used in my chapter in
Heiberger, Richard M., and Burt Holland (2008).
``Structured Sets of Graphs.''  Chapter III.6, (pp.~415--445) in
{\it Handbook of Computational Statistics on Data
Visualization}, edited by Chun-houh Chen, Antony Unwin, and Wolfgang
H\"{a}rdle.
Springer-Verlag, Berlin.

Rich

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