geom_rect: background color repeated per season - r

Question

I have a dataframe like this:
df<-data.frame(Category= c("a","b","a","b"), Value = c(25,90,40,10), Date= c("2016-02-13", "2016-05-13", "2016-08-13", "2016-11-13"))
In reality it is more complex, has several years and several observed objects so that it should be a faceted plot in the end, but I think this has nothing to do with the question.
I want to have a ggplot (line plot), where every season got it's own background color.
e.g.: spring from March to May in yellow,
summer from June to August in red
autumn from September to November in blue
and winter from December to February in grey.
This should be repeated, regardless the year as it goes through several years and the database will be updated with time.
I tried a lot with geom_rect but didn't find a working solution.
Thanks for any advice!

Answer 1

If I understand your goal correctly, I think you can achieve it by creating two additional variables, say Season and Color that correspond to Date column, and then supply the columns as necessary to geom_line.
To make the steps and the results clearer, I create a dummy data by expanding your data to another year (2017) with similar date and category but slightly different values:
1. The data
df<-data.frame(Category= c("a","b","a","b"), Value = c(25,90,40,10), Date= c("2016-02-13", "2016-05-13", "2016-08-13", "2016-11-13"))
df2<-data.frame(Category= c("a","b","a","b"), Value = c(30,95,45,15), Date= c("2017-02-13", "2017-05-13", "2017-08-13", "2017-11-13"))
dat <- rbind.data.frame(df,df2)
dat
Category Value Date
1 a 25 2016-02-13
2 b 90 2016-05-13
3 a 40 2016-08-13
4 b 10 2016-11-13
5 a 30 2017-02-13
6 b 95 2017-05-13
7 a 45 2017-08-13
8 b 15 2017-11-13
2. Creating Season and Color columns
dat.season <- dat %>%
mutate(Date = as.Date(Date)) %>%
mutate(Month = months(Date)) %>%
mutate(Season = case_when(Month %in% c("March", "April", "May") ~ "spring",
Month %in% c("June", "July", "August") ~ "summer",
Month %in% c("September", "October", "November") ~ "autumn",
Month %in% c("December", "January", "February")~ "winter")) %>%
mutate(Color = case_when(Season == "spring"~ "yellow",
Season == "summer"~ "red",
Season == "autumn"~ "blue",
Season == "winter"~ "grey"))
dat.season
Category Value Date Month Season Color
1 a 25 2016-02-13 February winter grey
2 b 90 2016-05-13 May spring yellow
3 a 40 2016-08-13 August summer red
4 b 10 2016-11-13 November autumn blue
5 a 30 2017-02-13 February winter grey
6 b 95 2017-05-13 May spring yellow
7 a 45 2017-08-13 August summer red
8 b 15 2017-11-13 November autumn blue
Supplying the columns to geom_line()
dat.season %>% ggplot() +
geom_line(aes(x = Date, y = Value),
colour = dat.season$Color) +
theme_bw()
The result
Update to add coloured background
Here is the line plot along with coloured backgrounds for each season.
dat.season %>%
ggplot() +
geom_rect(aes(xmin = Date[1], xmax = Date[2],
ymin = Value[1], ymax = Value[2]),
fill = dat.season$Color[1])+
geom_rect(aes(xmin = Date[2], xmax = Date[3],
ymin = Value[2], ymax = Value[3]),
fill = dat.season$Color[2])+
geom_rect(aes(xmin = Date[3], xmax = Date[4],
ymin = Value[3], ymax = Value[4]),
fill = dat.season$Color[3])+
geom_rect(aes(xmin = Date[4], xmax = Date[5],
ymin = Value[4], ymax = Value[5]),
fill = dat.season$Color[4])+
geom_rect(aes(xmin = Date[5], xmax = Date[6],
ymin = Value[5], ymax = Value[6]),
fill = dat.season$Color[5])+
geom_rect(aes(xmin = Date[6], xmax = Date[7],
ymin = Value[6], ymax = Value[7]),
fill = dat.season$Color[6])+
geom_rect(aes(xmin = Date[7], xmax = Date[8],
ymin = Value[7], ymax = Value[8]),
fill = dat.season$Color[7])+
geom_line(aes(x = Date, y = Value)) +
theme_bw()
The result