[R] Plot grouped data: How to change x-axis? (nlme)

[Karl Knoblick]

Hallo!

GENERAL QUESTION:
I'm trying to change the tick marks of the x-axis in a
grouped data plot (nlme). 

CONCRETE EXAMPLE:
In the example (see below) I want the x-axis to have
tick marks at 0, 6, 12, 18, 24. How can I do this? 

WHAT I TRIED
I tried "normal" methods like axis(...) but this does
not work with this plot. And I also tried xlim=c(0,24)
but the ticks are unchanged and the plot looks worse
(because of no distances to the border of the Panels).


EXAMPLE:
--
# begin example
library(lattice)
library(nlme)

# create sample data
set.seed(123)
ID<-rep(1:6, each=4)
Time<-rep(c(0,6,12,24), 6)
Y<-runif(24)
DF<-data.frame(ID, Time, Y)

GDF<-groupedData(Y ~ Time | ID, data=DF)

plot(GDF,layout=c(3,2), aspect=0.6, xlab="Months")

# end example

QUESTION: How can I get tick marks at 0, 6, 12, 18, 24
on the x-axis?

Any help will be appreciate.

Karl.

PS: Have a happy new year and keep R running!!