Use alpha for lines of geom_density(), geom_bar(), and so on

[yutannihilation]

Hi, thanks for this great package!

I saw many people (including me) are confused at the behavior of alpha when used in Geoms which draw areas or rects (e.g. geom_desity()). So, let me ask if this is your design decision or not.

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.

ggplot(diamonds, aes(carat)) +
    geom_density(fill = "red", colour = "red", alpha = 0.1)

I expect the one like this.

I know we can draw the latter chart by using colour=alpha("red", 0.1), but I believe alpha is a transparency param for the entire layer, not only for the line or the fill. For me, this is a matter of semantics. Are there any reason not to use alpha for colour?

The possible implementation is very simple. We just need to change those two lines to col = alpha(aes$colour, aes$alpha):

https://github.com/hadley/ggplot2/blob/bbc1b5ad3c0f55e8809d4fe1a9bd2783eb9fa29c/R/geom-rect.r#L55
https://github.com/hadley/ggplot2/blob/bbc1b5ad3c0f55e8809d4fe1a9bd2783eb9fa29c/R/geom-ribbon.r#L95

If this looks good to you, I'm happy to submit PR 😄

[hadley]

It was a deliberate design decision. But now I can't remember why 😦 I think I had a good reason though.

[yutannihilation]

Hmm..., thanks for your quick answer, anyway.

I found the behavior of alpha have been this way from the time it was born, but no background is described in the commit message.

36d5ad2

Okay, I trust in you this time. You always have a good reason; the very fact can be a good reason for me to close this issue 👍. Please close this issue anytime!

But, I hope you will occasionally remember that not a few people are confused at this. At least, this should be included in ggplot2 book.

[hadley]

You're right - it's more consistent to affect both. Done!

[yutannihilation]

Wow!!! Thanks for your decision! You are always great 👍

[antoine-lizee]

I am so sorry to see this! You should believe in yourself more hadley :-).
As a result, there is no way to separately tune the line and the fill without using two geoms now, one with fill = NA and another with color = NA. My very common use case is to have a transparent filing with a solid line - it is now impossible:

ggplot(diamonds %>% filter(cut %in% c("Very Good", "Ideal")), aes(x = price, fill = cut)) + theme_bw() + 
  geom_histogram(binwidth = 1000) +
  geom_density(aes(x = price, y = ..count.. * 1000), color = "black", alpha = 0.3) +
  geom_density(aes(x = price, y = ..count.. * 1000), color = "black", fill = NA) +
  facet_grid(cut ~ ., scales = "free_y")

While if you wanted to get the line lighter, you can always change its color!

Could we revert this? I'm pretty sure it's breaking more code now than it would if reverted for 2.1...

[hadley]

@antoine-lizee can you please file a new issue? I'm likely to lose track of an old issue with a comment.

[antoine-lizee]

Of course. #1523