gghighlight not working with ggplot faceting

I'm using ggplot and gghighlight. I have a dataframe with 2 continuous variables and 2 factors, and I wish to create a facetted plot and then highlight certain variable ranges using gghighlight.

library(ggplot2)
library(gghighlight)

n = 50
x = runif(n)
slope = sample(c(2,4), n, replace = TRUE)
intercept = sample(c(0, 0.5), n, replace = TRUE)
y <- slope * x + intercept

df <- data.frame(x = x, y = y, slope = slope, intercept = intercept)
df$slope <- as.factor(df$slope) 
df$intercept <- as.factor(df$intercept)

When I create the first example plot below, the highlighting works but the problem is that I can't see the unhighlighted variables that I want to be in dark grey

ggplot(df, aes(x, y)) +
  gghighlight(y < 1.5, unhighlighted_params = list(colour = "darkgrey")) +
  geom_line(aes(group = interaction(slope, intercept), colour = slope)) +
  geom_point(aes(shape = intercept, colour = slope)) +
  facet_wrap(~ slope)

When I move the gghighlight line down

ggplot(df, aes(x, y)) +
  geom_line(aes(group = interaction(slope, intercept), colour = slope)) +
  geom_point(aes(shape = intercept, colour = slope)) +
  gghighlight(y < 1.5, unhighlighted_params = list(colour = "darkgrey")) +
  facet_wrap(~ slope)

the unhighlighted data shows up in dark grey but up I am also getting all the data shown in each facet. Any advice would be much appreciated.

Solution

Using the option calculate_per_facet = TRUE:

(The default is FALSE because a common use case is to use each facet to highlight all the data in a specific group against the unhighlighted backdrop of the data for all other groups.)

library(ggplot2)
library(gghighlight)

n = 50
x = runif(n)
slope = sample(c(2,4), n, replace = TRUE)
intercept = sample(c(0, 0.5), n, replace = TRUE)
y <- slope * x + intercept

df <- data.frame(x = x, y = y, slope = slope, intercept = intercept)
df$slope <- as.factor(df$slope) 
df$intercept <- as.factor(df$intercept)

ggplot(df, aes(x, y)) +
  geom_line(aes(group = interaction(slope, intercept), colour = slope)) +
  geom_point(aes(shape = intercept, colour = slope)) +
  gghighlight(y < 1.5, unhighlighted_params = list(colour = "darkgrey"), 
              calculate_per_facet = TRUE
              ) +
  facet_wrap(~ slope)

^{Created on 2024-05-04 with reprex v2.1.0}