Plotting interactions among categorical variables in regression models

Simple two-way interaction

First, let’s prep some data. I’m going to make some slight changes to the mpg dataset from ggplot2 for didactic purposes to drop a few factor levels that have almost no values (e.g., there are 5 cylinder engines?).

library(ggplot2)
mpg2 <- mpg
mpg2$cyl <- factor(mpg2$cyl)
mpg2["auto"] <- "auto"
mpg2$auto[mpg2$trans %in% c("manual(m5)", "manual(m6)")] <- "manual"
mpg2$auto <- factor(mpg2$auto)
mpg2["fwd"] <- "2wd"
mpg2$fwd[mpg2$drv == "4"] <- "4wd"
mpg2$fwd <- factor(mpg2$fwd)
## Drop the two cars with 5 cylinders (rest are 4, 6, or 8)
mpg2 <- mpg2[mpg2$cyl != "5",]
## Fit the model
fit3 <- lm(cty ~ cyl * fwd * auto, data = mpg2)

So basically what we’re looking at here is an interaction between number of cylinders in the engine of some cars and whether the car has all-wheel drive or two-wheel drive. The DV is fuel mileage in the city.

Here’s summary output for our model:

library(jtools) # for summ()
summ(fit3)

Let’s see what happens using all the default arguments:

cat_plot(fit3, pred = cyl, modx = fwd)

This is with geom = "point". We can see a main effect of cyl and maybe something is going on with the interaction as well, since the different between 2wd and 4wd seems to decrease as cyl gets higher.

You can also plot the observed data on the plot:

cat_plot(fit3, pred = cyl, modx = fwd, plot.points = TRUE)

Line plots

And since cyl does have a clear order, it might make more sense to connect those dots. Let’s try geom = "line":

cat_plot(fit3, pred = cyl, modx = fwd, geom = "line")

Okay, that makes the trend quite a bit clearer.

You have some other options, too. Suppose you will need this plot to look good in black and white. Let’s change the shape of those points for different values of the moderator.

cat_plot(fit3, pred = cyl, modx = fwd, geom = "line", point.shape = TRUE)

You can change the line patterns as well for more clarity.

cat_plot(fit3, pred = cyl, modx = fwd, geom = "line", point.shape = TRUE,
         vary.lty = TRUE)

You may also choose any color palette from RColorBrewer as well as several preset palettes available in jtools:

cat_plot(fit3, pred = cyl, modx = fwd, geom = "line", point.shape = TRUE,
         colors = "Set2")

Use ?jtools_colors for more on your color options.

Bar/dynamite plots

Last but not least, you can also make bar charts, AKA dynamite plots. For many situations, these are not the best way to show your data, but I know it’s what a lot of people are looking for.

cat_plot(fit3, pred = cyl, modx = fwd, geom = "bar")

The transparency of the fill color depends on the presence of the error bars and observed data points.

cat_plot(fit3, pred = cyl, modx = fwd, geom = "bar", interval = FALSE)

Now let’s look with observed data:

cat_plot(fit3, pred = cyl, modx = fwd, geom = "bar", interval = FALSE,
         plot.points = TRUE)