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 = "boxplot", color = supp) + theme_bw()
p <- p + labs(x="Dose", y="Response")
p <- p + stat_summary(fun.y = mean, geom = "point", color = "blue")
p <- p + stat_summary(fun.y = mean, geom = "line", aes(group = 1))
p <- p  + opts(axis.title.x = theme_text(size = 12, hjust = 0.54, vjust = 0))
p <- p  + opts(axis.title.y = theme_text(size = 12, angle = 90,  vjust = 0.25))
print(p)

How can I plot dose-supp level combination means rather than only dose level means which I’m getting here? Thanks in advance for your help.

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).