r - Adding legend to ggplot when lines were added manually

Question

I have a plot in ggplot with 4 separate lines that I have added with a separate geom_line() argument. I would like to add legend but scale_colour_manual doesn't work in this case. What is the proper way to add legends when I added the variables separately?

Here's my code:

ggplot(proba[108:140,], aes(c,four)) + 
    geom_line(linetype=1, size=0.3) + 
    scale_x_continuous(breaks=seq(110,140,5)) + 
    theme_bw() + 
    theme(axis.line = element_line(colour = "black", size=0.25),
          panel.grid.major = element_blank(),
          panel.grid.minor = element_blank(),
          panel.border = element_blank(),
          panel.background = element_blank()) + 
    theme(axis.text.x = element_text(angle = 0, hjust = +0.5, size=6,color="black")) + 
    theme(axis.text.y = element_text(angle = 0, hjust = -100, size=6, color="black")) + 
    theme(axis.ticks=element_line(colour="black",size=0.25)) + 
    xlab("
Time-steps") + 
    ylab("Proportion correct
") + 
    theme(axis.text=element_text(size=8),axis.title=element_text(size=8)) + 
    geom_line(aes(c,three), size=0.2, linetype=2) + 
    geom_line(aes(c,one),linetype=3, size=0.8, colour="darkgrey") + 
    geom_line(aes(c,two), linetype=1, size=0.8, colour="darkgrey")

Answer 1

Just set the color name in aes to whatever the line's name on the legend should be.

I don't have your data, but here's an example using iris a line with random y values:

library(ggplot2)

line.data <- data.frame(x=seq(0, 10, length.out=10), y=runif(10, 0, 10))

qplot(Sepal.Length, Petal.Length, color=Species, data=iris) +
  geom_line(aes(x, y, color="My Line"), data=line.data)

The key thing to note is that you're creating an aesthetic mapping, but instead of mapping color to a column in a data frame, you're mapping it to a string you specify. ggplot will assign a color to that value, just as with values that come from a data frame. You could have produced the same plot as above by adding a Species column to the data frame:

line.data$Species <- "My Line"
qplot(Sepal.Length, Petal.Length, color=Species, data=iris) +
  geom_line(aes(x, y), data=line.data)

Either way, if you don't like the color ggplot2 assigns, then you can specify your own using scale_color_manual:

qplot(Sepal.Length, Petal.Length, color=Species, data=iris) +
  geom_line(aes(x, y, color="My Line"), data=line.data) +
  scale_color_manual(values=c("setosa"="blue4", "versicolor"="red4",
                              "virginica"="purple4", "My Line"="gray"))

Another alternative is to just directly label the lines, or to make the purpose of the lines obvious from the context. Really, the best option depends on your specific circumstances.