Adding text to multiple column graph (faceted) in ggplot2 - r

Question

I am trying to add p-values to each boxplot pair in the graph shown below. I would like the p-values to be placed under each soil horizon label ('O', 'A' and 'B').
My data looks like this:
> head(kiwi_l)
# A tibble: 6 x 6
type horizon root_name length diameter n_child
<chr> <chr> <chr> <dbl> <dbl> <int>
1 Elevated CO2 A R1_A_L_S4G 0.0752 0.0342 0
2 Elevated CO2 A R1_A_L_S4F 0.0987 0.0319 0
3 Elevated CO2 A R1_A_L_S4E 0.105 0.0209 0
4 Elevated CO2 A R1_A_L_S4D 0.0476 0.0127 0
5 Elevated CO2 A R1_A_L_S4C 0.110 0.0282 0
6 Elevated CO2 A R1_A_L_S4B 0.244 0.0168 0
While the code I used to generate the graph is:
l_horizon<-ggplot(kiwi, aes(x=type, y=length, fill=type, palette='jco'))
+
geom_boxplot() +
facet_grid(. ~ factor(horizon, level=level_order)) +
theme_pubr() +
scale_y_continuous(name='Primary root length (cm)') +
scale_x_discrete(name='Treatment') +
ggtitle('Soil horizon') + theme(plot.title = element_text(hjust = 0.5)) +
theme(legend.position="none") +
theme(plot.title = element_text(size = 10, face = "bold"),
text = element_text(size = 10),
axis.title = element_text(face="bold"),
axis.text.x=element_text(size = 10),
axis.text.y=element_text(size=10),
axis.title.x = element_blank(),
axis.title.y=element_text(size=10))
l_horizon<-l_horizon+scale_fill_locuszoom()
l_horizon
Please help!

Answer 1

Since there is no data to play around with, I'll make up some:
set.seed(0)
df <- data.frame(f1 = rep(c("O","A","B"), each = 30),
f2 = rep(c("M","N"), 45),
y = rnorm(90))
Next we do a test on that data and format it's output:
tests <- split(df, df$f1) %>% sapply(function(x){
pval <- t.test(x[x$f2 == "M", "y"], x[x$f2 == "N", "y"])$p.value
paste0("p-value = ", format(pval, digits = 2, nsmall = 2))
})
Now if you want it to be part of the facet strip, you can adjust the levels of df$f1 to include the p-value:
levels(df$f1) <- paste0(levels(df$f1), "\n", tests)
ggplot(df, aes(x = f2, y = y)) +
geom_boxplot() +
facet_grid(~ f1)
If you wanted the p-values inside the panel instead of in the strip, you can use the annotate() function to place them in the panel. y = Inf ensures they are placed at the top.
ggplot(df, aes(x = f2, y = y)) +
geom_boxplot() +
facet_grid(~ f1) +
annotate("text", x = 1.5, y = Inf, label = tests, vjust = 1)

Answer 2

If you know where the on the y-axes to put the text, maybe annotate like this?
p_values <- c(1.1,2.2,3.3)
ggplot(data = d2,mapping = aes(x=range,y=p_area)) +
geom_boxplot() +
annotate("text", x=c(1,2,3), y=0.5, label= p_values)