[R] Should nlme's augPred work with a factor covariate?

[Gavin Kelly]

I'm having difficulty getting the augPred function from the nlme
package to work when the primary covariate is a factor.  I don't know
if it is intended to work in these situations, but I can't immediately
see anything in the documentation that forbids this - ideally I'd like
to be able to plot the results just like I can for numeric primary
covariates.
Many thanks - Gavin Kelly (Cancer Research UK, Bioinformatics & Biostatistics)

> library(nlme)
> fm <- lme(ergoStool)
> augPred(fm)
Error in Summary.factor(c(1L, 2L, 3L, 4L, 1L, 2L, 3L, 4L, 1L, 2L, 3L,  :
  min not meaningful for factors

> sessionInfo()
R version 2.9.1 (2009-06-26)
i386-pc-mingw32

locale:
LC_COLLATE=English_United Kingdom.1252;LC_CTYPE=English_United
Kingdom.1252;LC_MONETARY=English_United
Kingdom.1252;LC_NUMERIC=C;LC_TIME=English_United Kingdom.1252

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base

other attached packages:
[1] nlme_3.1-92

loaded via a namespace (and not attached):
[1] grid_2.9.1      lattice_0.17-25