Unable to use fixed alpha aesthetic when color is mapped to a factor

[logworthy]

I'm trying to use a fixed alpha aesthetic to manually highlight points in a plot. It fails when color is mapped to a factor.

library(ggplot2)
data(mtcars)

# fails (but should work)
ggplot(mtcars, aes(x=disp, y=hp, color=as.factor(cyl)))+geom_point(alpha=seq(0,1,length.out=32))
# Error: Aesthetics must be either length 1 or the same as the data (3): alpha

# fails (and should fail)
ggplot(mtcars, aes(x=disp, y=hp, color=as.factor(cyl)))+geom_point(alpha=seq(0,1,length.out=3))
# Error: Aesthetics must be either length 1 or the same as the data (32): alpha

# succeeds
ggplot(mtcars, aes(x=disp, y=hp, color=cyl))+geom_point(alpha=seq(0,1,length.out=32))

Tested on version 2.2.0

[hadley]

This was a tricky one - it turns out the problem was with the legend.

[logworthy]

Thanks!