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In ggplot2, the layers are stacked in a particular order which determines which elements of the plot appear on top. By default, line segments created using stat_density_ridges() are placed behind the histograms. However, you can bring them to the front by changing the layer ordering.

One way to do this is to use the position_stack(reverse = TRUE) argument in ggplot() to reverse the order of the layers. You can adjust the z value of the line segments using the position argument in geom_density_ridges().

Here's an example:

library(ggplot2)
library(ggridges)

ggplot(mpg, aes(x = cty, y = class)) +
  geom_density_ridges(alpha = 0.5, color = "white", fill = "#69b3a2",
                       scale = 0.9, size = 0.25, position = position_stack(vjust = 0.5)) +
  geom_segment(aes(x = 0, xend = 40, y = class, yend = class), 
               size = 0.5, color = "black", position = position_stack(vjust = 0.5)) +
  scale_x_continuous(expand = c(0, 0)) +
  theme_ridges() +
  labs(x = NULL, y = NULL) +
  theme(legend.position = "bottom") +
  position_stack(reverse = TRUE)

In this example, we use position_stack(vjust = 0.5) in both geom_density_ridges() and geom_segments() to ensure that they are aligned vertically. We set position_stack(reverse = TRUE) in ggplot() to reverse the order of the layers, so that the line segments appear on top of the histograms. Finally, we adjust the z value of the line segments using position = position_stack(vjust = 0.5).

The result is a plot where the line segments appear in front of the histograms: