Remove extra legends in ggplot2

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?

Aesthetics can be set or mapped within a ggplot call.

• An aesthetic defined within aes(...) is mapped from the data, and a legend created.
• An aesthetic may also be set to a single value, by defining it outside aes().

In this case, it appears you wish to set alpha = 0.8 and map colour = group.

To do this,

Place the alpha = 0.8 outside the aes() definition.

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

For any mapped variable you can supress the appearance of a legend by using guide = 'none' in the appropriate scale_... call. eg.

g2 <- ggplot(df, aes(x = x, y = y, group = group)) + 
        geom_line(aes(colour = group)) +
        geom_point(aes(colour = group, alpha = 0.8))
g2 + scale_alpha(guide = 'none')

Which will return an identical plot

EDIT @Joran's comment is spot-on, I've made my answer more comprehensive

Just add the show.legend = F code after the part where you don't want it.

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), show.legend = F)

For old versions of ggplot2 (versions before 0.9.2, released in late 2012), this answer should work:

I tried this with a colour_scale and it did not work. It appears that the colour_scale_hue item works like a function with a default parameter TRUE. I added scale_colour_hue(legend=FALSE) and it worked.

I am not sure if this is the case for all color scale items in ggplot