Introduce new alpha parameter for lines of geom_density(), geom_bar(), and so on #3112
Description
(It's my first time, and I am really sorry if I waste someones time to close and delete this, but I am kinda confident, that it is the right place.)
I open this issue (again), because as I read it, the last time no real solution was agreed upon, and the confusing behavior is not (easily) resolved. The issue in question can be found here:
The discussion is very informative, so I will only quote the basic problem:
"When I draw a chart with alpha, I expect that the whole layer, the line and fill, will become transparent. But the line stays clear red."
I would argue, that this is extermely counterintuitive, and apparently some people agree (See link).
Because (of course) some peoples code would be broken by a proposed change, the suggested change has been reverted, and the workaround is to change the color manually with colour=alpha("red", 0.1).
The problem with this solution, is that I can not longer use automatic color picking to highlight a group with color=MyGroupVar, alpha=ifelse(MyGroupVar=="GroupA",0.2,1). I personally think it is very inconvenient, but as I understand the debate, a possible compromise would be to introduce a new parameter alpha_line or something.
Or am I missing a very simple solution for this? (In any case something more "official" than just a workaround of any kind would be nice ^^)
Below is a simple example:
mtcars$cyl = as.factor(mtcars$cyl)
ggplot(data=mtcars, aes(x=disp,fill=cyl, color=cyl, alpha=ifelse(cyl!=6,0.2,1))) +
geom_bar(width=5,position=position_dodge(preserve = "single"))+
guides(alpha=FALSE)
