Allow select alpha to be applied on fill (default), colour, or both

[atusy]

This PR is related to #1371

In the discussion in #1371, a change was attempted to apply alpha both on colour and fill.
In this PR, we can select alpha to be applied on fill (default), colour, or both.
The default selection is fill and thus is safe in terms of backward compatibility.

Currently, alpha_to is implemented for geom_polygon, geom_rect, and geom_ribbon.
It works in some more (e.g., geom_density), but not all (geom_bar).
So I need some more to work on.

Before I update other functions, I'd like to ask @hadley if you like my PR.

Examples

Varying alpha_to by fill, colour, and both

library(ggplot2)
d <- data.frame(xmin = 0, xmax = 1, ymin = 0:4, ymax = 1:5)
g <- ggplot(d) +
  aes(
    xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax,
    colour = ymin, alpha = ymin
  )
g + geom_rect(size = 3) # alpha_to = "fill"

g + geom_rect(size = 3, alpha_to = "colour")

g + geom_rect(size = 3, alpha_to = "both")

^{Created on 2019-08-15 by the reprex package (v0.3.0)}

[clauswilke]

I feel certain this is the wrong API. It's better (and easier to implement!) to set up a hierarchy of aesthetics. This is done e.g. in ggridges, where point_alpha can be used to override the application of alpha specifically to points.

library(ggplot2)
library(ggridges)

# without `point_alpha`
ggplot(iris, aes(x=Sepal.Length, y=Species)) +
  geom_density_ridges(
    jittered_points = TRUE,
    position = position_points_jitter(width = 0.05, height = 0),
    point_shape = '|', point_size = 3, alpha = 0.4,
  )
#> Picking joint bandwidth of 0.181

# with `point_alpha`
ggplot(iris, aes(x=Sepal.Length, y=Species)) +
  geom_density_ridges(
    jittered_points = TRUE,
    position = position_points_jitter(width = 0.05, height = 0),
    point_shape = '|', point_size = 3, point_alpha = 1, alpha = 0.4,
  )
#> Picking joint bandwidth of 0.181

^{Created on 2019-08-15 by the reprex package (v0.3.0)}

Whether we want this in ggplot2 proper is a separate question though. I don't see a strong need. For all geoms provided by ggplot2 one can always achieve the same effect by using two separate geoms (e.g., geom_area() and geom_line()).

Another relevant discussion: #1523

[atusy]

better (and easier to implement!) to set up a hierarchy of aesthetics

Sound like you are right.
Your idea is much more flexible because alpha of color and fill can be based on different variables.

achieve the same effect by using two separate geoms

For geom_density, its true that geom_area() and geom_line() works.
What about for geom_rect?
Is there any geom which just draws border of rectangles with a support of alpha?
Maybe geom_path is the one, but is not as easy as using geom_rect.

[yutannihilation]

Maybe geom_path is the one, but is not as easy as using geom_rect.

But, it's still possible. I agree it would be nice to have some option to make alpha affect on lines, but I don't see a good reason to do so, considering the difficulty of making changes on these fundamental geoms and maintaining them. As we saw on #1371, I think it's too late to change....

[atusy]

I see...

[yutannihilation]

You can create an extension package, though.

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