customizing the output from `plot_interaction()`

In modsem: Latent Interaction (and Moderation) Analysis in Structural Equation Models (SEM)

EVAL_DEFAULT <- FALSE
knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>",
  eval = TRUE
)

Since plot_interaction() just returns a ggplot object, the output be customized using a combination of different ggplot2 functions.

Baseline Plot

Here we fit a simple model, and create a baseline plot, which we'll customize.

library(modsem)
library(ggplot2)

m <- '
X =~ x1 + x2 + x3
Z =~ z1 + z2 + z3
Y =~ y1 + y2 + y3

Y ~ X + Z + X:Z
'

fit <- modsem(m, data = oneInt, method = "lms")
p <- plot_interaction(x = "X", z = "Z", y = "Y", vals_z = c(-1, 1), model = fit)
print(p)

Changing Titles and Labels

Title

The title of the plot can be changed using the ggtitle() function.

p + ggtitle("This is a new title")

Axis Labels

The axis labels can be changed using the xlab() and ylab() functions.

p + xlab("This is the x-axis") + ylab("This is the y-axis")

Legend Title

Changing the legend title is a bit trickier, but it can be done using the guides() function. Since plot_interaction() uses both the fill and color aesthetic, both must be specified, with the same title.

p + guides(fill=guide_legend(title = "New Legend Title"),
           color=guide_legend(title = "New Legend Title"))

Changing Colors

The colors of both the fill and color aesthetic can be changed using different functions.

Greyscale

Here we replace the fill aesthetic with a greyscale.

p + scale_fill_grey()

Here we do the same for the color aesthetic.

p + scale_color_grey()

Combining both, we get a greyscale version of the plot.

p + scale_color_grey() + scale_fill_grey()

NOTE: As of version 1.0.14 there is a separate greyscale argument, for using a greyscale theme, simplifying the process.

Brewer Color palettes

ggplot2 comes with a few different color palettes, which we can use to the replace the default color palette. Here we have numerous options (you can also download additional packages), but here we can see a few examples using some of the brewer palettes.

To see the available palettes you can run this snippet.

# You might need to install the `RColorBrewer` package
RColorBrewer::display.brewer.all()

Here we have an example using the Dark2 palette.

# We need to apply the color scale to both the fill and color aesthetics
p + scale_fill_brewer(palette = "Dark2") +
    scale_color_brewer(palette = "Dark2")

Here we have an example using the Accent palette.

# We need to apply the color scale to both the fill and color aesthetics
p + scale_fill_brewer(palette = "Accent") +
    scale_color_brewer(palette = "Accent")

Custom Color Palettes

It is also possible to use custom/manual color palettes, but this is more cumbersome. Here we see an example.

custom_colors = c("#E69F00", "#56B4E9")

p + scale_fill_manual(values = custom_colors) +
    scale_color_manual(values = custom_colors)

Changing Line types

Changing the linetypes is a more compliacted than changing the colors in the plot. This is because we have to add a whole new aesthetic to the plot. This can done using the aes() function. Behind the scenes, modsem uses a variable cat_z to form the groups in the output from plot_interaction, which is why we pass linetype = cat_z.

p + aes(linetype = cat_z)

Here we can see that we had an unfortunate side effect, where we created a second Legend in our plot. This can however be fixed by renaming the Legend title (see the [Legend Title] section).

p + aes(linetype = cat_z) +
    guides(linetype = guide_legend("Z"))

Combinations

Here we can see an example combining the different options.

p +
  ggtitle("Customized Plot") + # New Title
  xlab("Values of X") + # New X-axis label
  ylab("Predicted values of Y") + # New y-axis label
  scale_color_grey() + # Add Grey theem
  scale_fill_grey() +
  aes(linetype = cat_z) + # Add linetypes
  guides(color    = guide_legend("Values of Z"), # Change Legend Title
         fill     = guide_legend("Values of Z"),
         linetype = guide_legend("Values of Z"))

modsem documentation built on Nov. 19, 2025, 1:07 a.m.