I needed to produce a few a interaction plots for my book in R and, while the interaction.plot() function is useful it has a couple of drawbacks. First, the default output isn’t very pretty. Second, it works from the raw data, whereas I often need plots from cell means. For teaching purposes it is quite common to produce plots without raw data (for hypothetical data or from published examples).

My first attempts at the plots involved setting them up element by element. Just going over some examples I decided to turn the basic plot (for a 2 x 2 ANOVA) into a simple function. Nothing fancy, just a regular interaction plot in black and white that I think is prettier than the SPSS, Excel or R defaults. At some point I may have a go turning it into a general I x J ANOVA plot (or maybe even add CIs, but I’ll probably do that from raw data if I ever get round to it).

plot.2by2 <- function(A1B1,A1B2, A2B1, A2B2, group.names, legend = TRUE, leg.loc=NULL, factor.labels=c('Factor A', 'Factor B'), swap = FALSE, ylab= NULL, main = NULL){
group.means <- c(A1B1, A2B1, A1B2, A2B2)
if(missing(ylab)) ylab <- expression(italic(DV))
if(swap==TRUE) {
group.names <- list(group.names[[2]], group.names[[1]]) ; group.means <- c(A1B1, A1B2, A2B1, A2B2); factor.labels <- c(factor.labels[2], factor.labels[1])
}
plot(group.means, pch=NA, ylim=c(min(group.means)*.95, max(group.means)*1.025), xlim=c(0.8,2.2), ylab=ylab, xaxt=’n’, xlab=factor.labels[1], main=main)
points(group.means[1:2], pch = 21)
points(group.means[3:4], pch = 19)
axis(side = 1, at = c(1:2), labels = group.names[[1]])
lines(group.means[1:2], lwd = .6, lty = 2)
lines(group.means[3:4], lwd = .6)
if(missing(leg.loc)) leg.loc <- c(1,max(group.means))
if(legend ==TRUE) legend(leg.loc[1], leg.loc[2],legend = group.names[[2]],  title = factor.labels[2], lty = c(3,1))
}

lev.names <- list(c('A1', 'A2'), c('B1', 'B2'))
plot.2by2(5,15,10,20, lev.names)

You can swap the axes by adding the argument swap = TRUE:

plot.2by2(5,15,10,20, lev.names, swap = TRUE)

The default factor names are ‘Factor A’ and ‘Factor B’, but these are over-ridden in the call:

plot.2by2(5,15,10,20,lev.names, swap = TRUE, factor.labels= c(‘Factor 1′,’Factor 2’))


You can also change the y-axis label with ylab or add a main title with main.  The legend can be dropped (legend = FALSE) if you don’t want one or need it to be located outside the plot. To move the legend just specify coordinates with an argument such as leg.loc = c(1,10). You can also edit the source code directly. Here is an example with title and meaningful labels:


group.names <- list(c('placebo','drug'), c('male', 'female'))

plot.2by2(10,10,15,20, group.names, factor.labels=c(‘Drug’, ‘Sex’), swap = FALSE)