Interaction Plot in ggplot2

Question

I\'m trying to make interaction plot with ggplot2. My code is below:

library(ggplot2)
p <- qplot(as.factor(dose), len, data=ToothGrowth, geom         


        

Answer 1

You can precalculate the values in their own data frame:

toothInt <- ddply(ToothGrowth,.(dose,supp),summarise, val = mean(len))

ggplot(ToothGrowth, aes(x = factor(dose), y = len, colour = supp)) + 
    geom_boxplot() + 
    geom_point(data = toothInt, aes(y = val)) +
    geom_line(data = toothInt, aes(y = val, group = supp)) + 
    theme_bw()

Note that using ggplot rather than qplot makes the graph construction a lot clearer for more complex plots like these (IMHO).