How to use loop with geom_vline and facet_wrap?

I have data similar to the one I’ve created below:

set.seed(42)
dates <- seq.Date(as.Date("2012-08-01"), as.Date("2014-08-30"), "day")
n <- length(dates)
dat <- data.frame(date = dates,
                  category = rep(LETTERS[1:4], n/2),
                  daily_count = sample(18:100, n, replace=TRUE))

#following to be used for creating dotted lines; highlighting a certain point for each category
point_dates <- sample(seq.Date(as.Date("2012-08-01"), as.Date("2014-08-30"), "month"),4)
category_name <- list("A", "B", "C", "D")

I am creating a boxplot for each category using facet_wrap, and point_dates are important for me as they shows the point of interest in each boxplot. This is how I am creating the plot:

ggplot(dat) +
  geom_boxplot(aes(y = daily_count,
                   x = yearmonth(date),
                   group = paste(yearmonth(date), category),
                   fill = category)) +
  labs(x = 'Month & Year',
       y = 'Count',
       fill = "Category") +
  theme_bw() +
  theme(axis.text=element_text(size=10),
        axis.title=element_text(size=10),
        legend.position="none") +
  geom_vline(xintercept =  lubridate::ymd("2013-08-23"), linetype=1, colour="red", size = 0.5)+
  
  sapply(point_dates[[1]], function(xint) geom_vline(data=filter(dat, 
  category==category_name[[1]]),aes(xintercept = xint),
  linetype=3, colour="black", size = 1))+
  
  sapply(point_dates[[2]], function(xint) geom_vline(data=filter(dat, 
  category==category_name[[2]]),aes(xintercept = xint),
  linetype=3, colour="black", size = 1))+
  
  sapply(point_dates[[3]], function(xint) geom_vline(data=filter(dat, 
  category==category_name[[3]]),aes(xintercept = xint),
  linetype=3, colour="black", size = 1))+
  
  sapply(point_dates[[4]], function(xint) geom_vline(data=filter(dat, 
  category==category_name[[4]]),aes(xintercept = xint),
  linetype=3, colour="black", size = 1))+
  
  facet_wrap(~category, nrow = 2)

And this is the output of the code:

The plot is being created just fine. My question is, is there any better way (loop may be?) that would help me get rid of writing sapply multiple times. Because the number of categories may change (increase/decrease), that would be to change the code everytime.

Any guidance please?

>Solution :

I’m not sure that this is the best way, but you could do all of them in one go using map2 from tidyr. This would save you time from having to write out individual sapply.

library(tidyverse)

ggplot(dat) +
  geom_boxplot(aes(y = daily_count,
                   x = yearmonth(date),
                   group = paste(yearmonth(date), category),
                   fill = category)) +
  labs(x = 'Month & Year',
       y = 'Count',
       fill = "Category") +
  theme_bw() +
  theme(axis.text=element_text(size=10),
        axis.title=element_text(size=10),
        legend.position="none") +
  geom_vline(xintercept =  lubridate::ymd("2013-08-23"), 
             linetype=1, colour="red", size = 0.5)+
  map2(point_dates, category_name, 
       ~geom_vline(data=filter(dat, category==.y),
                   aes(xintercept = .x),
                   linetype=3, colour="black", size = 1))+
  facet_wrap(~category, nrow = 2)