The default values for plotting a factor x are the integers
1:length(levels(x)). This class and its functions provide a way of
specifying alternate plotting locations for the levels.
A virtual Class: No objects may be created from it.
Class "ordered", directly.
Class "factor", by class "ordered", distance 2.
Class "oldClass", by class "ordered", distance 3.
No methods defined with class "positioned" in the signature.
S3-type methods are
"[.positioned",
as.double.positioned,
as.numeric.positioned,
as.positioned,
is.numeric.positioned,
is.positioned,
positioned,
print.positioned,
unique.positioned.
Although interaction.positioned should be a method, it isn't
because interaction is not a generic and can't easily be made one
since the name interaction.plot conflicts.
Heiberger, Richard M. and Holland, Burt (2004b). Statistical Analysis and Data Display: An Intermediate Course with Examples in S-Plus, R, and SAS. Springer Texts in Statistics. Springer. ISBN 0-387-40270-5.