Remove extra legends in ggplot2

Question

I have a simple data frame that I'm trying to do a combined line and point plot using ggplot2. Supposing my data looks like this:

df <- data.frame(x=rep(1:10,2), y=c(1:10,11:20), 

                 group=c(rep("a",10),rep("b",10)))

And I'm trying to make a plot:

g <- ggplot(df, aes(x=x, y=y, group=group))

g <- g + geom_line(aes(colour=group))

g <- g + geom_point(aes(colour=group, alpha = .8))

g

The result looks fine with one exception. It has an extra legend showing the alpha for my geom_point layer.

How can I keep the legend showing group colors, but not the one that shows my alpha settings?

Answer 1

To remove the extra legend from your plot, you need to place the alpha argument outside the aesthetic function since aesthetic defined within aes(...) is mapped from the data, and a legend is created. And it appears that you wish to set alpha = 0.8 and map colour = group.i.e.,

df <- data.frame(x=rep(1:10,2), y=c(1:10,11:20), 

                 group=c(rep("a",10),rep("b",10)))

g <- ggplot(df, aes(x = x, y = y, group = group))

g <- g + geom_line(aes(colour = group))

g <- g + geom_point(aes(colour = group), alpha = 0.8)

g

Output: