Horizontal axis title on top after coord_flip

I've been trying for a while now to put the horizontal axis title on top of the chart after using coord_flip() in ggplot2, but without any success.

I know I could:

1. Set scale_y_continuous (position = "right")
2. Set theme (axis.title.x.top = element_text (face = "bold")) # I guess any format would do here, right?

But then:

1. The whole axis would go to the top, and I want only the axis title there
2. It doesn't work, for some reason

CODE

example <- iris |> 

ggplot             (aes (x = reorder (Species, Petal.Width), y = Petal.Width))                          +    
geom_bar           (stat = "identity", fill = "gray60")                                                 +
geom_hline         (yintercept = 100 / nrow (iris), linetype = "dashed", color = "firebrick", size = 1) +
scale_y_continuous (limits = c(0, 15), breaks = seq (0, 15, 2.5), expand = c(0, 0))                     +
coord_flip         ()                                                                                   +

labs (x = "", y = "Horizontal axis title") +

theme (axis.title       = element_text (family = "MS", face = "bold", size = 18, color = "black"),
       axis.text        = element_text (family = "TNR", face = "plain", size = 16, color = "black"),
       axis.text.x      = element_text (angle = 45),
       panel.background = element_rect (fill = NA),
       panel.grid.major = element_line (color = "gray90"),
       panel.grid.minor = element_line (color = "gray90"),
       plot.margin      = margin       (t = 20, r = 20, b = 5, l = 5)) +

windows (); example

I'd defer to @stefan's solution for splitting up the axis title and the breaks; that is simplest with a duplicate axis.

I would add, though, that most cases to use coord_flip() were eliminated with ggplot2 3.3.0 in 2020. (Though occasionally you may need to add the orientation parameter if the aesthetics don't make it clear).

Then we can do:

iris |> 
  ggplot(aes(y = reorder (Species, Petal.Width), x = Petal.Width))  +    
  geom_col(fill = "gray60") +
  geom_vline(xintercept = 100 / nrow (iris), linetype = "dashed", 
             color = "firebrick", size = 1) +
  scale_x_continuous(limits = c(0, 15), breaks = seq (0, 15, 2.5), 
                      expand = c(0, 0), name = NULL,
                      sec.axis = dup_axis(breaks = NULL, name = "Horizontal axis title")) +
  labs(y = NULL) # avoids even having a space for the axis title