Using a color scheme and legend labels in plot_model

Question

I would like to plot the results of a multilevel model in R, specifically to illustrate an interaction effect, using plot_model from sjPlot. I would like to use a specific color scheme and I would also like to label the three levels of the legend (low, medium, high). However, when I try to do both of these things, the shading reverts to the default color scheme.

Here is a reproducible example of what I've tried, using a merTools dataset.

library(lme4)
library(sjPlot)
library(merTools)

colors<-c("red","darkred","pink")

m1 = lmer(mathach ~ ses + meanses + ses*meanses + (1|schid), data=hsb)

p1<-plot_model(m1, type="pred", terms= c("ses","meanses"), 
               colors= colors, legend.title = "Mean SES")+
  labs(y="SES", x="Mean SES", 
       linetype = "meanses") +
  scale_color_manual(labels = c("Low (-1 SD)", "Average", "High (+1 SD)"), values=colors) 
p1

This gets me the message "Scale for colour is already present. Adding another scale for colour, which will replace the existing scale."

If I leave out the scale_color_manual code, I get the coloring that I wanted, but not the legend labels I wanted.

Answer 1

To add your labels and apply your colors for both the color and the fill aesthetic add aesthetics = c("color", "fill") to scale_color_manual.

Using a minimal reproducible example based on the default example from ?lme4::lmer:

library(lme4)
#> Loading required package: Matrix

library(sjPlot)
library(ggplot2)

colors <- c("red", "darkred", "pink")

sleepstudy$foo <- rep(LETTERS[1:3], each = 60)

m1 <- lmer(Reaction ~ Days + Days * foo + (Days | Subject), sleepstudy)

p1 <- plot_model(m1,
  type = "pred", terms = c("Days", "foo"), 
  colors = colors, legend.title = "Mean SES"
) +
  labs(
    y = "SES", x = "Mean SES",
    linetype = "meanses"
  ) +
  scale_color_manual(
    labels = c("Low (-1 SD)", "Average", "High (+1 SD)"), values = colors,
    aesthetics = c("color", "fill")
  )
#> Scale for colour is already present.
#> Adding another scale for colour, which will replace the existing scale.

p1