ggplot-empty-levels.r

library(tidyverse)
d <- tibble(group = letters[1:5], x = 1:5, y = rnorm(5))
ggplot(d, aes(x, y, colour = group)) + geom_point(size = 3)

# lose group
d <- d[-2,]
ggplot(d, aes(x, y, colour = group)) + geom_point(size = 3)

# fix by joining in all known groups to create NAs:
all_groups <- tibble(group = letters[1:5])
d <- full_join(d, all_groups) # <- key part here
d
ggplot(d, aes(x, y, colour = group)) + geom_point(size = 3)

# or more elegantly, define the factor levels and use drop = FALSE:
d <- d[-2,] # lose group
d$group <- factor(d$group, levels = c("a", "b", "c", "d", "e"))
ggplot(d, aes(x, y, colour = group)) + geom_point(size = 3) +
  scale_color_discrete(drop = FALSE)