Add symbol on top of ggplot2 boxplots to indicate value of variable - r

Question

Working with the following subset of a much larger dataset,
ex <- structure(list(transect_id = c(1L, 1L, 1L, 1L, 1L, 15L, 15L,
15L, 15L, 15L, 15L), number_f = c(2L, 2L, 2L, 2L, 2L, 0L, 0L,
0L, 0L, 0L, 0L), years_f = c(1L, 1L, 1L, 1L, 1L, 6L, 6L, 6L,
6L, 6L, 6L), b = c(5.036625862, 6.468666553, 8.028989792, 4.168409348,
5.790089607, 10.67796993, 9.371051788, 10.54364777, 6.904324532,
7.203606129, 9.1611166)), .Names = c("transect_id", "number_f",
"years_f", "b"), class = "data.frame", row.names = c(1L, 2L,
3L, 4L, 5L, 2045L, 2046L, 2047L, 2048L, 2049L, 2050L))
I've plotted the distributions of "b" for each of the groups indicated by "transect_id" and have colored them by "number_f", which I do here:
ggplot(aes(x=reorder(transect_id, b, FUN=median), y=b), data=ex) + geom_boxplot(aes(fill=as.factor(number_f))) + xlab('Transect ID')
What I need to do for each of the "transect_id" groups is stack symbols - asterisks or some other symbol - on top of each boxplot to provide an indication of the value of "years_f" that corresponds to each "transect_id". In the data subset below, "years_f" amounts to 1 and 6 for transect_ids 1 and 15, respectively. I'd like to see something like this, which I manually mocked up.
Also keep in mind that the dataset I'm working with is very large so I'll need to use some loop or some other way of doing this automatically. Please note that I absolutely welcome other ideas for better ways of indicating the value of "years_f" that might not overburden the figure as much as having all of these stacked symbols that will particularly be an issue for larger values of "years_f".

Answer 1

Try adding
annotate('text', x = c(1, 2), y = 3, label = paste0('Year_F =', unique(ex$years_f)))
to the end of your plot like so:
ggplot(aes(x=reorder(transect_id, b, FUN=median), y=b), data=ex) +
geom_boxplot(aes(fill=as.factor(number_f))) + xlab('Transect ID')+
annotate('text', x = c(1, 2), y = 3, label = paste0('Year_F =', unique(ex$years_f)))
To use it on a bigger dataset you would have to edit the x and y argument, but this might be a decent alternative. A possibility for the y coordinate could be something like 0.9 * min(ex$b).
edit In response to your comment:
You could first count how many levels there are of transect_id to specify x
len.levels <- length(levels(as.factor(ex$transect_id)))
then, you could create a summary table of the uniqe years_f variable by transect_id:
sum.table <- aggregate(years_f~reorder(ex$transect_id, ex$b, median),
data = ex, FUN = unique)
reorder(ex$transect_id, ex$b, median) years_f
1 1 1
2 15 6
and then plot as follows:
ggplot(aes(x=reorder(transect_id, b, FUN=median), y=b), data=ex) +
geom_boxplot(aes(fill=as.factor(number_f))) + xlab('Transect ID')+
annotate('text', x = 1:len.levels, y = .9 * min(ex$b),
label = paste0('Year_F =', sum.table[,2]))