ggplot2 - Adding a traditional legend to dumbbell plot in `ggalt::geom_dumbbell` in `R`

Question

How to add a traditional legend to dumbbell plot created using ggalt::geom_dumbbell in R?

This question has an answer with an in-chart legend. How to map the aesthetics to get a separate legend for the points on the side/bottom ?

library(ggalt)

df <- data.frame(trt=LETTERS[1:5], l=c(20, 40, 10, 30, 50), r=c(70, 50, 30, 60, 80))

ggplot(df, aes(y=trt, x=l, xend=r)) + 
  geom_dumbbell(size=3, color="#e3e2e1", 
                colour_x = "red", colour_xend = "blue",
                dot_guide=TRUE, dot_guide_size=0.25) +
  theme_bw()

Answer 1

One way to get a legend is to add a points layer based on the dataset in long format, mapping color to the grouping variable.

First, make a long format dataset via gather from tidyr.

df2 = tidyr::gather(df, group, value, -trt)

Then make the plot, adding the new points layer with the long dataset and using scale_color_manual to set colors. I moved the geom_dumbbell specific aesthetics into that layer.

ggplot(df, aes(y = trt)) + 
     geom_point(data = df2, aes(x = value, color = group), size = 3) +
     geom_dumbbell(aes(x = l, xend = r), size=3, color="#e3e2e1", 
                   colour_x = "red", colour_xend = "blue",
                   dot_guide=TRUE, dot_guide_size=0.25) +
     theme_bw() +
     scale_color_manual(name = "", values = c("red", "blue") )