How to add a line to a continuous color legend but not the plot?

Question

Is there a way to add a line just to a continuous gradient legend? I was unable to find any examples of this using ggplot2.

For example, how could I add a r

Answer 1

Not sure how to add a custom line on gradient legend, but I do know how to add a custom tick label with custom color:

library(ggplot2)
ggplot(df, aes(x, y, color = y)) + 
  geom_point() +
  scale_colour_gradient(breaks = c(-2, 0, 1.7, 2),
                        labels = c(-2, 0, "1.7 (important)", 2)) +
  guides(color = guide_colorbar(barheight = 10,
                                label.theme = element_text(colour = c("black", "black", 
                                                                      "red", "black"), 
                                                           angle = 0,
                                                           size = 12)))

Notice that the "1.7" label overlaps the "2" label. You can either do something like the following:

ggplot(df, aes(x, y, color = y)) + 
  geom_point() +
  scale_colour_gradient(breaks = c(-2, 0, 1.7, 2),
                        labels = c(-2, 0, "<-- 1.7 (important)", 2)) +
  guides(color = guide_colorbar(barheight = 10,
                                label.theme = element_text(colour = c("black", "black", 
                                                                      "red", "black"), 
                                                           angle = 0,
                                                           size = 12)))

or adjust the horizontal position of the "1.7" label:

ggplot(df, aes(x, y, color = y)) + 
  geom_point() +
  scale_colour_gradient(breaks = c(-2, 0, 1.7, 2),
                        labels = c(-2, 0, "1.7 (important)", 2)) +
  guides(color = guide_colorbar(barheight = 10,
                                label.hjust = c(0, 0, 0.1, 0),
                                label.theme = element_text(colour = c("black", "black", 
                                                                      "red", "black"), 
                                                           angle = 0,
                                                           size = 12)))