ggplot Adding manual legend to plot without modifying the dataset

Question

I am trying to add a manual legend to a plot without modifying the dataset, because the dataset and lines that mark mean, median etc. are different concepts.

Approaches to solve the problem modifying the data exist, e.g. Adding manual legend to a ggplot

Here is a simplified example, the real dataset is more complex:

vec <-c(rep(1,100),rep(2,100),rep(3,80),rep(4,70),rep(5,60))
tbl <- as_tibble(vec)

mean_vec <- mean(vec)

cols <- c("Frequency"="grey",
          "mean"="blue")

ggplot(as_tibble(tbl)) + 
  aes(x = value) +
  geom_histogram(binwidth=1) +
  geom_vline(xintercept=mean_vec,col="blue",size=1)+
  theme_minimal() +
  scale_color_manual(values=cols)+
  scale_fill_manual(name="Test",values=cols)

Why is the legend missing in the plot?

score 2 · Accepted Answer · answered Apr 18 '22 at 07:26

If you want to have a legend then you have to map on aesthetics. Otherwise scale_color/fill_manual will have no effect:

vec <- c(rep(1, 100), rep(2, 100), rep(3, 80), rep(4, 70), rep(5, 60))
tbl <- data.frame(value = vec)

mean_vec <- mean(vec)

cols <- c(
  "Frequency" = "grey",
  "mean" = "blue"
)

library(ggplot2)

ggplot(tbl) +
  aes(x = value) +
  geom_histogram(aes(fill = "Frequency"), binwidth = 1) +
  geom_vline(aes(color = "mean", xintercept = mean_vec), size = 1) +
  theme_minimal() +
  scale_color_manual(values = "blue") +
  scale_fill_manual(name = "Test", values = "grey")

ggplot Adding manual legend to plot without modifying the dataset

1 Answers1