Highlight minimum and maximum points in faceted ggplot2 graph in R

Question

I am using the built-in economics (from the ggplot2 package) dataset in R, and have plotted a time-series for each variable in the same graph using

Answer 1

Method 1: Using Joins

This can be nice when you want to save the filtered subsets

---

library(reshape2)
library(ggplot2)
library(dplyr)

me <- melt(economics, id=c("date"))

me %>%
  group_by(variable) %>%
  summarise(min = min(value),
            max = max(value)) -> me.2

left_join(me, me.2) %>%
  mutate(color = value == min | value == max) %>%
  filter(color == TRUE) -> me.3

ggplot(data=me, aes(x = date, y = value)) + 
  geom_line() +
  geom_point(data=me.3, aes(x = date, y = value), color = "red") +
  facet_wrap(~variable, ncol=1, scales='free_y')

Method 2: Simplified without Joins

Thanks @Gregor

me.2 <- me %>%
  group_by(variable) %>%
  mutate(color = (min(value) == value | max(value) == value))

ggplot(data=me.2, aes(x = date, y = value)) +
  geom_line() +
  geom_point(aes(color = color)) +
  facet_wrap(~variable, ncol=1, scales="free_y") +
  scale_color_manual(values = c(NA, "red"))