Add means to a boxplot - r

Question

I want to add the mean values to my basic boxplot and found this function here.
fun_mean <- function(x){
return(data.frame(y=mean(x),label=mean(x,na.rm=T)))
}
I used it within my code, but because I have two factors, it is not working properly. Where do I have to add the second factor?
FixationT2.plot = ggplot(dataT2fix_figs,
aes(x = length, y = perc_fixated, fill = mask)) +
geom_boxplot() +
coord_cartesian (ylim =c(35, 100)) +
geom_hline(yintercept = 50) +
stat_summary(fun.y = mean, geom="point", colour="darkred", size=3) +
labs(title="") +
xlab("Länge Wort N+1") +
ylab("Fixationswahrscheinlichkeit in %\n von Wort N+1") +
guides(fill=guide_legend(title="Preview Maske"))
This is what the data looks like
Subject length mask perc_fixated
<fct> <fct> <fct> <dbl>
1 1 "kurzes\n N+1" keine Maske 41.7
2 1 "kurzes\n N+1" syntaktisch korrekt 91.7
3 1 "kurzes\n N+1" syntaktisch inkorrekt 86.7
4 1 "langes \nN+1" keine Maske 100
5 1 "langes \nN+1" syntaktisch korrekt 87.5
6 1 "langes \nN+1" syntaktisch inkorrekt 91.7
7 2 "kurzes\n N+1" keine Maske 73.3
8 2 "kurzes\n N+1" syntaktisch korrekt 84.6
9 2 "kurzes\n N+1" syntaktisch inkorrekt 83.3
10 2 "langes \nN+1" keine Maske 83.3

Answer 1

You can specify the dodge width for the calculated mean value layer. Right now they appear to be overlapping one another at each x-axis value. I don't see the function you mentioned (fun_mean) actually used in the ggplot code, but it shouldn't really be necessary.
Try this:
ggplot(df,
aes(x = length, y = perc_fixated, fill = mask)) +
geom_boxplot() +
stat_summary(fun.y = mean, geom="point", colour="darkred", size=3,
position = position_dodge2(width = 0.75))
# ... code for axis titles & so on omitted for brevity.
I used width = 0.75 above, because this is the default width for geom_boxplot() / stat_boxplot() (as found in the ggplot2 code here). If you specify a width explicitly in your boxplot, use that instead.
Data used:
df <- read.table(header = TRUE,
text = 'Subject length mask perc_fixated
1 1 "kurzes\n N+1" "keine Maske" 41.7
2 1 "kurzes\n N+1" "syntaktisch korrekt" 91.7
3 1 "kurzes\n N+1" "syntaktisch inkorrekt" 86.7
4 1 "langes \nN+1" "keine Maske" 100
5 1 "langes \nN+1" "syntaktisch korrekt" 87.5
6 1 "langes \nN+1" "syntaktisch inkorrekt" 91.7
7 2 "kurzes\n N+1" "keine Maske" 73.3
8 2 "kurzes\n N+1" "syntaktisch korrekt" 84.6
9 2 "kurzes\n N+1" "syntaktisch inkorrekt" 83.3
10 2 "langes \nN+1" "keine Maske" 83.3')
df$Subject <- factor(df$Subject)
(Next time, please use dput() as advised in the comments to provide your data.)

Answer 2

I have in the past just used the points() function to add mean to my box plots like this:
boxplot(mtcars$mpg ~ mtcars$cyl)
points(x = c(1, 2, 3),
y = tapply(mtcars$mpg, mtcars$cyl, "mean"), col = "red")
So you plot a boxplot then calculate the mean for each of your boxes and plot them as the y argument in points and the x is just a sequence with length how ever many boxes you have.