How can I add a legend to a ggplot in R? [duplicate] - r

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Plotting two variables as lines using ggplot2 on the same graph
(5 answers)
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I'm using a data frame in R with 3 variables. I want to plot (ggplot) 2 variables (CMod4X and CMod5X) as two distinct lines, in function of the 3th variable (AmtX). In the end I succeed in creating some kind of graph that suits me, but I fail to include a legend. I have already consulted some other treads here, but the answers don't seem not to work for me.
The (artificial) data set looks like this
AmtX <- seq(from = 1, to = 10001, by = 50)
CMod4X <- rnorm(201, mean = 0.87, sd = 0.01)
CMod5X <- rnorm(201, mean = 0.84, sd = 0.01)
EvalAmtX <- as.data.frame(cbind(AmtX,CMod4X,CMod5X))
I have made the plot like this
pltX <- ggplot(data = EvalAmtX, aes (x = AmtX)) +
geom_line(aes(y = CMod4X), color = "red", show.legend = TRUE) +
geom_line(aes(y = CMod5X), color = "blue", show.legend = TRUE) +
geom_smooth(aes(y = CMod4X), color = "red", se = FALSE, show.legend = TRUE) +
geom_smooth(aes(y = CMod5X), color = "blue", se = FALSE, show.legend = TRUE) +
labs(y = "C-index", x = "Amount (Tau)", title = "model 4 and model 5") +
scale_colour_manual(name = "Models", values = c("CMod4" = "red", "CMod5" = "blue"))
pltX
But this plot won't include a label. I've included my plot below:
What am I doing wrong and what must I do to obtain a plot telling me the red line is CMod4 and the blue line is CMod5?
Thx for your help!!
Leonard

Answer 1

I guess you need to dive a little deeper into how ggplot2 works, since your question is related to the basic set up of your data frame. There are a lot of great resources around on this topic, e.g. this one. Anyway, here are two solutions for putting the legend into your graph.
Solution 1: Rearrange data frame to long format
library(reshape2)
df <- melt(data = EvalAmtX, id.vars = "AmtX")
The data frame now looks like this:
head(df)
# AmtX variable value
# 1 1 CMod4X 0.8772716
# 2 51 CMod4X 0.8524197
# 3 101 CMod4X 0.8686019
# 4 151 CMod4X 0.8638835
# 5 201 CMod4X 0.8674627
# 6 251 CMod4X 0.8729925
Now, plotting is easy. Instead of telling ggplot2 the color of each individual line, you simply give it the information which column in your data frame contains the factor that should determine the color of the lines. So you add another aesthetic (col = variable). This also automatically adds a legend for color.
ggplot(df, aes(x=AmtX, y=value, col = variable)) +
geom_line()
Solution 2: Use a manual color scale
You almost got it right in your code.
pltX <- ggplot(data = EvalAmtX, aes (x = AmtX)) +
geom_line(aes(y = CMod4X, color = "CMod4")) +
geom_line(aes(y = CMod5X, color = "CMod5")) +
geom_smooth(aes(y = CMod4X, color = "CMod4"), se = FALSE) +
geom_smooth(aes(y = CMod5X, color = "CMod5"), se = FALSE) +
labs(y = "C-index", x = "Amount (Tau)", title = "model 4 and model 5") +
scale_colour_manual(name = "Models", values = c(CMod4 = "red", CMod5 = "blue"))
pltX