R: Interaction Plot with a continuous and a categorical variable for a GLMM (lme4)

Question

I would like to make an interaction plot to visually display the difference or similarity in slopes of interaction of a categorical variable (4 levels) and a standardized co

Answer 1

Here's an answer of sorts (by the way, you had some missing quotation marks in your data frame above, which had to be fixed manually ...)

Fit the model:

library(lme4)
fit <- glmer(resp.var ~ cont.var:cat.var + (1|rand.eff) ,
           data = sample.data , poisson)

(Note that this is a slightly weird model specification -- forces all categories to have the same value at cont.var==0. Did you mean cont.var*cat.var?

library(ggplot2)
theme_update(theme_bw())  ## set white rather than gray background

Quick and dirty linear regressions:

ggplot(sample.data,aes(cont.var,resp.var,linetype=cat.var))+
    geom_smooth(method="lm",se=FALSE)

Now with a Poisson GLM (but not incorporating the random effect), and showing the data points:

ggplot(sample.data,aes(cont.var,resp.var,colour=cat.var))+
    stat_sum(aes(size=..n..),alpha=0.5)+
    geom_smooth(method="glm",family="poisson")

The next bit requires the development (r-forge) version of lme4, which has a predict method:

Set up data frame for prediction:

predframe <- with(sample.data,
                  expand.grid(cat.var=levels(cat.var),
                              cont.var=seq(min(cont.var),
                              max(cont.var),length=51)))

Predict at population level (REform=NA), on the linear predictor (logit) scale (this is the only way you will get straight lines on the plot)

predframe$pred.logit <- predict(fit,newdata=predframe,REform=NA)

minmaxvals <- range(sample.data$cont.var)

ggplot(predframe,aes(cont.var,pred.logit,linetype=cat.var))+geom_line()+
    geom_point(data=subset(predframe,cont.var %in% minmaxvals),
               aes(shape=cat.var))

Now on the response scale:

predframe$pred <- predict(fit,newdata=predframe,REform=NA,type="response")
ggplot(predframe,aes(cont.var,pred,linetype=cat.var))+geom_line()+
    geom_point(data=subset(predframe,cont.var %in% minmaxvals),
               aes(shape=cat.var))