R ggplot2 - geom_segment + facet_grid does not line up correctly

Question

I can't seem to get the geom_segement to match up when add facet_grid

library(tidyverse)
library(plotly)

#create data frame in long format
df <- mtcars %>% rownames_to_column(var = "car") %>% 
  select(car, mpg, disp, cyl) %>% 
  mutate(car = as.factor(car)) %>% 
  pivot_longer(cols = c(mpg, disp)) %>% 
  rowwise() %>% 
  mutate(target = runif(1, value*.8, value*1.2))

#create ggplot 
   ggplot(df) +
    scale_x_discrete() +
     geom_col(aes(x = car, y = value)) +
     geom_segment(
       aes(
         x = as.numeric(car)-.25,
         xend = as.numeric(car)+.25,
         y = target,
         yend = target
       ),
       linetype = 'dotted',
       color = 'black'
     ) +
      facet_grid(
        rows = vars(cyl),
        cols = vars(name),
        scales = "free",
        space = "free_y"
      ) +
      coord_flip()

As you can see below the 'reference lines' I'm creating with geom_segement aren't correct, they should be over the geom_col()

Answer 1

It seems to work if you use scales = "free_x" instead of scales = "free":

See also In ggplot2, coord_flip and free scales don't work together.

library(ggplot2)
library(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
library(tibble)
library(tidyr)

#create data frame in long format
df <- mtcars %>% rownames_to_column(var = "car") %>% 
    select(car, mpg, disp, cyl) %>% 
    mutate(car = as.factor(car)) %>% 
    pivot_longer(cols = c(mpg, disp)) %>% 
    rowwise() %>% 
    mutate(target = runif(1, value*.8, value*1.2))

#create ggplot 
ggplot(df) +
    # scale_x_discrete() +
    geom_col(aes(x = car, y = value)) +
    geom_segment(
        aes(
            x = as.numeric(car)-.25,
            xend = as.numeric(car)+.25,
            y = target,
            yend = target
        ),
        linetype = 'dotted',
        color = 'black'
    ) +
    facet_grid(
        rows = vars(cyl),
        cols = vars(name),
        scales = "free_x",
        space = "free_y"
    ) +
    coord_flip()

^{Created on 2020-06-04 by the reprex package (v0.3.0)}