[R] plot groupedData in nlme

[Qiong Yang]

Hi,

Does anyone know how to make the color of the lines all black when 
plotting groupedData with an outer factor:

For example,
library(nlme)
plot(Dialyzer, outer=~QB, key=F)

This generated colored curves in R.2.4.1. How to make all the curves black ?
(or how to alter the color (type) of lines for the nlme groupedData 
plotting function in general?)

Thanks
Qiong