Annotate ggplot2 across multiple facets - r

Question

I have recently started using the facet_nested function from the ggh4x package and I really like the look of the nested axis. I would like to annotate the plot to show stats that I have run. I have created a dummy dataset to illustrate my problem.
library(tidyverse)
library(markdown)
library(ggtext)
library(ggh4x)
df <- data.frame(pretreatment = c("10NA", "10NA","10NA", "NT", "NT", "NT"),
timepoint = c("0 h", "6 h","6 h", "0 h", "6 h", "6 h"),
treatment = c("baseline", "10NA", "NT","baseline", "10NA", "NT"),
mean_copy_no = c(1000, 1500, 1200, 600, 700, 400),
sample_id = c(1, 2, 3, 4, 5, 6))
df %>%
ggplot(aes(x=sample_id, y = mean_copy_no, fill = treatment)) +
geom_col(colour = "black") +
facet_nested(.~ pretreatment + timepoint + treatment, scales = "free", nest_line = TRUE, switch = "x") +
ylim(0,2000) +
theme_bw() +
theme(strip.text.x = element_text(size = unit(10, "pt")),
legend.position = "none",
axis.title.y = element_markdown(size = unit(13, "pt")),
axis.text.y = element_text(size = 11),
axis.text.x = element_blank(),
axis.title.x = element_blank(),
axis.ticks.x = element_blank(),
strip.text = element_markdown(size = unit(12, "pt")),
strip.background = element_blank(),
panel.spacing.x = unit(0.05,"line"),
panel.grid.major.x = element_blank(),
panel.grid.minor.x = element_blank(),
panel.border = element_blank())
This generates the following plot.
Now my problem is that each of the bars is located within its own facet and not all on one x axis (if you run the code without the theme, it shows it more clearly).
I have drawn how I would like the plot to look.
I would like to add lines and stars to indicate significant differences.
I can easilly add the stars, however I am struggling to add the lines and I understand that this may not even be possible because I am using facets to generate the plot. I just wanted to post the question and see if someone has any suggestion on how to do it in R. Or if there is a way to achieve the nested look without using facets.
*Edited for clarity.

Answer 1

One option is to use cowplot after making the ggplot object, where we can add the lines and text.
library(ggplot2)
library(cowplot)
results <- df %>%
ggplot(aes(x=sample_id, y = mean_copy_no, fill = treatment)) +
geom_col(colour = "black") +
facet_nested(.~ pretreatment + timepoint + treatment, scales = "free", nest_line = TRUE, switch = "x") +
ylim(0,2000) +
theme_bw() +
theme(strip.text.x = element_text(size = unit(10, "pt")),
legend.position = "none",
axis.title.y = element_markdown(size = unit(13, "pt")),
axis.text.y = element_text(size = 11),
axis.text.x = element_blank(),
axis.title.x = element_blank(),
axis.ticks.x = element_blank(),
strip.text = element_markdown(size = unit(12, "pt")),
strip.background = element_blank(),
panel.spacing.x = unit(0.05,"line"),
panel.grid.major.x = element_blank(),
panel.grid.minor.x = element_blank(),
panel.border = element_blank())
ggdraw(results) +
draw_line(
x = c(0.07, 0.36),
y = c(0.84, 0.84),
color = "black", size = 1
) +
annotate("text", x = 0.215, y = 0.85, label = "*", size = 15) +
draw_line(
x = c(0.7, 0.98),
y = c(0.55, 0.55),
color = "black", size = 1
) +
annotate("text", x = 0.84, y = 0.56, label = "**", size = 15)
Output