Create arbitrary legend ordering for multilayer ggplot2 plot

Question

I mocked up the following code to provide context. The functions, sequence intervals, names, and colors are all arbitrary.

library(ggplot2)

function1 <- function(input) {
  input * 3
}

function2 <- function(input) {
 2 * input + 1
}

function3 <- function(input) {
  input + 4
}

x1 <- seq(1, 10, 0.1)
x2 <- seq(1, 10, 0.2)
x3 <- seq(1, 10, 0.5)

y1 <- sapply(x1, function1)
y2 <- sapply(x2, function2)
y3 <- sapply(x3, function3)

data1 <- data.frame(x1, y1)
data2 <- data.frame(x2, y2)
data3 <- data.frame(x3, y3)

ggplot() + 
  geom_point(data = data1, aes(x1, y1, color = "B")) +
  geom_point(data = data2, aes(x2, y2, color = "C")) +
  geom_point(data = data3, aes(x3, y3, color = "A")) +
  scale_color_manual(name = "Functions", 
                     values = c("B" = "Green", "C" = "Red", 
                                "A" = "Blue")) +
  xlab("X") +
  ylab("Y")

Here is a screenshot of the resulting plot:

There are a few previously answered questions that address similar issues with legend ordering, such as this one , but none seem to deal with multilayer plots. This question addresses ordering of legends for version 0.9.2, but ggplot2 is currently on version 2.2.1 . Additionally, it seems to address only ascending or descending orders.

I'd like to know if there's any way to customize the order of the values in the legend . For example, in the legend, is it possible to display it as B, C, A instead of A, B, C?

Answer 1

The "ggplot2 way" would be to reshape your data to long format (or create it in long format in the first place). Then you need only one call to geom_point and you can create a factor column to order the functions:

dat = data.frame(X=c(x1,x2,x3), 
                 Y=c(y1,y2,y3), 
                 Functions=rep(LETTERS[1:3], sapply(list(x1,x2,x3), length)))

dat$Functions = factor(dat$Functions, levels=c("B","C","A"))

ggplot(dat, aes(X, Y, colour=Functions)) +
  geom_point() +
  scale_color_manual(values=c(B="green", C="red", A="blue")) 

UPDATE: In response to the comment, if you want to add an abline, you can use the code below. However, this will not only add a new key value to the colour legend, it will also add a diagonal line to the other three pre-existing legend keys.

ggplot(dat, aes(X, Y, colour=Functions)) +
  geom_point() +
  scale_color_manual(values=c(B="green", C="red", A="blue", `My Abline`="black")) +
  geom_abline(aes(intercept=0, slope=1, colour="My Abline"))

If you want a separate legend for the abline, then you could use a fill aesthetic for the points and reserve the colour legend only for the abline. To do this, use a filled point marker (point marker shapes 21 through 25). In the code below, stroke=0 is to remove the border around the filled points.

ggplot(dat, aes(X, Y, fill=Functions)) +
  geom_point(shape=21, size=2, stroke=0) +
  geom_abline(aes(intercept=0, slope=1, colour="My Abline")) +
  scale_fill_manual(values=c(B="green", C="red", A="blue")) +
  scale_colour_manual(values="black") +
  labs(colour="")