unexpected behavior for alpha in geom_rect() -- possible bug

[rpruim]

I'm trying to figure out why alpha is ignored by geom_rect() in the second example below. Is this a bug?

library(ggplot2)
# this works as expected
p <- ggplot() + geom_point(aes(y = eruptions, x = waiting), data = faithful)
p

p + geom_rect(aes(ymin = 2, ymax = 3, xmin = 50, xmax = 80), alpha = 0.2, 
              fill = "red")

# this ignores alpha for some reason
q <- 
  ggplot(aes(y = eruptions, x = waiting), data = faithful) + 
  geom_point()
q

q + geom_rect(aes(ymin = 2, ymax = 3, xmin = 50, xmax = 80), alpha = 0.2, 
              fill = "red")

p$mapping
#> Aesthetic mapping: 
#> <empty>
q$mapping
#> Aesthetic mapping: 
#> * `x` -> `waiting`
#> * `y` -> `eruptions`

Created on 2018-07-31 by the reprex package (v0.2.0).

[hadley]

You’re plotting one rectangle for each row 😉

[rpruim]

OK. I guess that makes sense.

But supposing q already exists (perhaps created by some slick plotting routine in an extension package) and I want to add just one rectangle. What's the easiest solution? I tried data = NULL and data = NA.

I confirmed that this works:

q + geom_rect(aes(ymin = 2, ymax = 3, xmin = 50, xmax = 80), alpha = 0.2,
                fill = "red", data = faithful[1,])

And if I don't have access to the data used to construct the plot? Then I guess I can (a) make up a meaningless data frame with 1 row and (b) use inherit.aes = FALSE.

Is there something more elegant?

[rpruim]

Interestingly, I see that a data frame with 0 rows and 0 columns does produce 1 rectangle:

q + geom_rect(aes(ymin = 2, ymax = 3, xmin = 50, xmax = 80), alpha = 0.2,
              fill = "red", data = data.frame(), inherit.aes = FALSE)

So that's better than making up data that don't get used. Is that my best option?

[hadley]

Yup.

[lock]

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