About adding/changing colors in ggplot2 (geom_hline)

Question

data %>% ggplot() + 
  geom_hline(aes(yintercept=Trial, col=Participant2)) + 
  scale_color_discrete(name="NI-1", labels=c("False Alarm", "Hit", "Miss")) + 
  theme_minimal()

Is there a way to assign the following colors to the labels in the line of code above?

False alarm- red

Hit- white

Miss- blue.

Thank you!

Answer 1

We may use scale_colour_manual() and assign custom colours and labels to the various levels of our discrete variable.

Here I use data from iris as example data and iris$Species as discrete variable.

library(ggplot2)

# axis labels
xl <- "Sepal Length"; yl <- "Sepal Width"

# custom colours
my_colours <- c('blue4', 'darkorange', '#00b0a6')
my_colours <- setNames(my_colours, unique(iris$Species))

# custom labels
my_labels <- c('species 1', 'species 2', 'species 3')

# median Sepal.Width per Species
medians_SW <- sapply(split(iris, iris$Species), \(x) median(x$Sepal.Width))

# plot
ggplot(iris, aes(x = Sepal.Length, y = Sepal.Width, colour = Species))  +
  geom_point(size = 2, alpha = 0.8) +
  geom_hline(aes(yintercept = medians_SW[1]), color = my_colours[1],
             linetype = 2, size = 0.8, alpha = 0.8) +
  geom_hline(aes(yintercept = medians_SW[2]), color = my_colours[2],
             linetype = 2, size = 0.8, alpha = 0.8) +
  geom_hline(aes(yintercept = medians_SW[3]), color = my_colours[3],
             linetype = 2, size = 0.8, alpha = 0.8) +
  xlab(xl) +  ylab(yl) + theme_minimal() +
  scale_colour_manual(name = 'Species',
                      breaks = unique(iris$Species),
                      values = my_colours,
                      labels = my_labels)