Using geom_rect for time series shading in R

Its a bit easier using annotate and also note that the bounds for the rectange can be specified as shown:

ggplot(a_series_df, aes(month, big)) + 
    geom_line() +
    annotate("rect", fill = "red", alpha = 0.5, 
        xmin = 1924, xmax = 1928 + 11/12,
        ymin = -Inf, ymax = Inf) +
    xlab("time")

This would also work:

library(zoo)

z <- read.zoo(a_series_df, index = 2)
autoplot(z) + 
    annotate("rect", fill = "red", alpha = 0.5, 
        xmin = 1924, xmax = 1928 + 11/12,
        ymin = -Inf, ymax = Inf) + 
    xlab("time") +
    ylab("big")

Either one gives this:

Code works fine, conversion to decimal date is needed for xmin and xmax, see below, requires lubridate package.

library("lubridate")
library("ggplot2")

ggplot(a_series_df)+
  geom_line(mapping = aes_string(x = "month", y = "big")) +
  geom_rect(
    fill = "red", alpha = 0.5, 
    mapping = aes_string(x = "month", y = "big"), 
    xmin = decimal_date(as.Date(c("1924-01-01"))),
    xmax = decimal_date(as.Date(c("1928-12-31"))),
    ymin = 0,
    ymax = 2
  )

Cleaner version, shading plotted first so the line colour doesn't change.

ggplot() +
  geom_rect(data = data.frame(xmin = decimal_date(as.Date(c("1924-01-01"))),
                              xmax = decimal_date(as.Date(c("1928-12-31"))),
                              ymin = -Inf,
                              ymax = Inf),
            aes(xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax),
            fill = "grey", alpha = 0.5) +
  geom_line(data = a_series_df,aes(month, big), colour = "blue") +
  theme_classic()