# How to map more informative values onto fill argument of sjPlot::plot_model

Whereas the direction of main effects can be interpreted from the sign of the estimate, the interpretation of interaction effects often requires plots. This task is facilitated by the R package `sjPlot`

. For instance, using the `plot_model`

function, I plotted the interaction between a continuous variable and a categorical variable. The categorical variable was passed to the `fill`

argument of plot_model.

```
library(lme4)
#> Loading required package: Matrix
library(sjPlot)
#> Install package "strengejacke" from GitHub (`devtools::install_github("strengejacke/strengejacke")`) to load all sj-packages at once!
library(ggplot2)
theme_set(theme_sjplot())
cake$recipe_recoded = ifelse(cake$recipe == 'A', -0.5,
ifelse(cake$recipe == 'B', 0,
ifelse(cake$recipe == 'C', 0.5,
NA)))
fit = lmer(angle ~ recipe_recoded * temp +
(1|recipe_recoded:replicate),
cake, REML= FALSE)
plot_model(fit, type = 'pred', terms = c('temp', 'recipe_recoded'))
#> Warning: Ignoring unknown parameters: linewidth
```

^{Created on 2023-06-24 with reprex v2.0.2}

However, I needed an extra feature, as the categorical variable was not quite informative because it was a sum-coded transformation. Thus, I wanted the legend of the plot to show the values of the original variable (i.e., A, B and C), instead of those of the sum-coded variable that had been used in the model (i.e., -0.5, 0 and 0.5).

Below is a solution using a custom function called `alias_interaction_plot`

.

```
library(lme4)
#> Loading required package: Matrix
library(sjPlot)
library(ggplot2)
theme_set(theme_sjplot())
cake$recipe_recoded = ifelse(cake$recipe == 'A', -0.5,
ifelse(cake$recipe == 'B', 0,
ifelse(cake$recipe == 'C', 0.5,
NA)))
fit = lmer(angle ~ recipe_recoded * temp +
(1|recipe_recoded:replicate),
cake, REML= FALSE)
# plot_model(fit, type = 'pred', terms = c('temp', 'recipe_recoded'))
# Displaying the original variable instead
# Read in function from GitHub
source('https://raw.githubusercontent.com/pablobernabeu/language-sensorimotor-simulation-PhD-thesis/main/R_functions/alias_interaction_plot.R')
alias_interaction_plot(
model = fit,
dataset = cake,
x = 'temp',
fill = 'recipe_recoded',
fill_alias = 'recipe',
fill_title = 'recipe'
)
#> Loading required package: rlang
#> Loading required package: dplyr
#>
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#>
#> filter, lag
#> The following objects are masked from 'package:base':
#>
#> intersect, setdiff, setequal, union
#> Loading required package: RColorBrewer
#> Loading required package: ggtext
#> Loading required package: Cairo
#> Warning: Ignoring unknown parameters: linewidth
#> Scale for 'y' is already present. Adding another scale for 'y', which will
#> replace the existing scale.
#> Scale for 'colour' is already present. Adding another scale for 'colour',
#> which will replace the existing scale.
```

^{Created on 2023-06-24 with reprex v2.0.2}