I'm fitting a lme growth curve model with two predictors and their
interaction as predictors. The multilevel model is nested so that level 1
is time within the individual, and level 2 is the individual.  I would like
to plot the mean group-level trajectories at plus and minus 1 SD from the
mean of the main effects composing the interaction term using ggplot.
 Thus, the plot should have 4 lines (mean trajectories) representing all
combinations of the two predictors (where LO = -1sd from its mean, and HI =
+1sd from its mean):
-LO predictor, LO moderator
-LO predictor, HI moderator
-HI predictor, LO moderator
-HI predictor, HI moderator

They are growth curves, so the plot should have 'time' on the x-axis and
'outcome' on the y-axis.  Here's a sample model:

library(nlme)

set.seed(123)

dataset <- NULL
dataset$id <- rep(1:100, each=10)
dataset$time <- rep(1:10,100)
dataset$outcome <- rnorm(1000, mean=100, sd=15)
dataset$predictor <- rnorm(1000, mean=10, sd=3)
dataset$moderator <- rnorm(1000, mean=5, sd=2)

dataset <- as.data.frame(dataset)
dataset[sample(1:1000, 400, replace=F),"outcome"] <- NA
dataset[sample(1:1000, 400, replace=F),"predictor"] <- NA
dataset[sample(1:1000, 400, replace=F),"moderator"] <- NA

model <- lme(outcome~ time + predictor + moderator + predictor*moderator,
random=~1 + time + predictor|id, data=dataset, na.action=na.omit)

How can I plot the interaction on the mean group-level trajectories using
ggplot?  I'm new to ggplot, so any help would be greatly appreciated.  Thanks
in advance!

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