Change line size of a single plot inside facet_grid

Question

I'd like to change the size of a particular line, inside a single facet_grid, and keep the others unchanged. This in order to "highlight" more a line.

Fake data:

set.seed(123)
my_data <- data.frame(
  time = 1:100,
  a = runif(100, min = 0, max = 10),
  b = runif(100, min = 0, max = 20),
  c = runif(100, min = 0, max = 30)
)


library(ggplot2)
library(dplyr)
my_data %>% 
  gather("key", "value", -time) %>%
  ggplot(aes(x = time, y = value, color = key)) +
  geom_line() +
  facet_grid(key~., scales = "free") +
  theme_minimal() +
  guides(color = FALSE, size = FALSE)

In this example, I'd like the b plot to have a bigger line size.

Answer 1

This can be achieved by creating a new vector having the repeated sizes:

linesize = rep(c(0, 1, 0), each=100) # externally define the sizes
# note that a,c will have size=0 while b size=1

This will be used inside the geom_line call:

my_data %>% 
  gather("key", "value", -time) %>%
  ggplot(aes(x = time, y = value, color = key)) +
  geom_line(size = linesize) + # here we can pass linesize
  facet_grid(key~., scales = "free") +
  theme_minimal() +
  guides(color = FALSE)