How can I add a layer showing the distribution on a conditional variable in a probability plot in R studio?

Question

I am fitting the following regression: model <- glm(DV ~ conditions + predictor + conditions*predictor, family = binomial(link = "probit"), data = d) .

I use 'sjPlot' (and 'ggplot2') to make the following plot:

library("ggplot2")
library("sjPlot")
plot_model(model, type = "pred", terms = c("predictor", "conditions")) +
  xlab("Xlab") +
  ylab("Ylab") +
  theme_minimal() +
  ggtitle("Title")>

But I can't figure out how to add a layer showing the distribution on the conditioning variable like I can easily do by setting "hist = TRUE" using 'interplot':

library("interplot")
interplot(model, var1 = "conditions", var2 = "predictor", hist = TRUE) +
      xlab("Xlab") +
      ylab("Ylab") +
      theme_minimal() +
      ggtitle("Title") 

I have tried a bunch of layers using just ggplot as well, with no success

ggplot(d, aes(x=predictor, y=DV, color=conditions))+
  geom_smooth(method = "glm") +
  xlab("Xlab") +
  ylab("Ylab") +
  theme_minimal() +
  ggtitle("Title")

.

I am open to any suggestions!

Answer 1

I've obviously had to try to recreate your data to get this to work, so it won't be faithful to your original, but if we assume your plot is something like this:

p <- plot_model(model, type = "pred", terms = c("predictor [all]", "conditions")) +
  xlab("Xlab") +
  ylab("Ylab") +
  theme_minimal() +
  ggtitle("Title")

p

Then we can add a histogram of the predictor variable like this:

p + geom_histogram(data = d, inherit.aes = FALSE, 
                   aes(x = predictor, y = ..count../1000),
                   fill = "gray85", colour = "gray50", alpha = 0.3)

And if you wanted to do the whole thing in ggplot, you need to remember to tell geom_smooth that your glm is a probit model, otherwise it will just fit a normal linear regression. I've copied the color palette over too for this example, though note the smoothing lines for the groups start at their lowest x value rather than extrapolating back to 0.

ggplot(d, aes(x = predictor, y = DV, color = conditions))+
  geom_smooth(method = "glm", aes(fill = conditions),
              method.args = list(family = binomial(link = "probit")),
              alpha = 0.15, size = 0.5) +
  xlab("Xlab") +
  scale_fill_manual(values = c("#e41a1c", "#377eb8")) +
  scale_colour_manual(values = c("#e41a1c", "#377eb8")) +
  ylab("Ylab") +
  theme_minimal() +
  ggtitle("Title") + 
  geom_histogram(aes(y = ..count../1000),
                 fill = "gray85", colour = "gray50", alpha = 0.3)

---

Data

set.seed(69)

n_each     <- 500
predictor  <- rgamma(2 * n_each, 2.5, 3)
predictor  <- 1 - predictor/max(predictor)
log_odds   <- c((1 - predictor[1:n_each]) * 5 - 3.605, 
              predictor[n_each + 1:n_each] * 0 + 0.57)
DV         <- rbinom(2 * n_each, 1, exp(log_odds)/(1 + exp(log_odds)))
conditions <- factor(rep(c("  ", " "), each = n_each))
d          <- data.frame(DV, predictor, conditions)