Change ggplot legend title

Question

So this is my code for my ggplot. How do I easiest change the title of the legend? I know that I can just change my gg_group variable to my_title <- c(rep("train",10), rep("validation", 10)). But i want to just change the title to "whatever I want" without change any variables.

library(ggplot2)
y <- c(rnorm(10,1), rnorm(10,3))
x <- rep(seq(1,10,1),2)
gg_group <- c(rep("train",10), rep("validation", 10))

gg_data <- data.frame(y=y, x=x, gg_group=gg_group)

p <- ggplot(gg_data, aes(x=x, y=y, group=gg_group))
p + geom_line(aes(colour=gg_group))

I have also tried this code:

p + geom_line(aes(colour=gg_group)) + scale_shape_discrete(name="Dataset",labels=c("Train", "Validation"))

But this does not work. *Edit, check great snwer from Jaap and JasonAizkalns.

Answer 1

@Jaap is correct. If you use scale_color_discrete you can change the name of the legend with name and you do not have to pass any arguments to labels as they will assume the names defined in your colour aesthetic. That is consider the differences between:

p + geom_line(aes(colour = gg_group)) +
  scale_color_discrete(name = "Dataset")

and

p + geom_line(aes(colour = gg_group)) +
  scale_color_discrete(name = "Dataset", 
                       labels = c("New Label 01", "New Label 02"))

Answer 2

The reason it is not working is because you did not use a shape in your ggplot code. Instead you should use scale_color_discrete as follows:

scale_color_discrete("Dataset")