Should alpha() support patterned fills?

[teunbrand]

Since R 4.2.0, the {grid} package has supported vectorised patterns. A simplified example from the link, with a list of patterns, is the code below:

library(grid)

grad <- linearGradient(
  c(palette()[2], "white"), 
  y1 = 0.5, y2 = 0.5, group = FALSE
)
pat  <- pattern(
  circleGrob(r = unit(2, "mm"), gp = gpar(col = 2, fill = 2)),
  width = unit(5, "mm"), height = unit(5, "mm"),
  extend = "repeat", group = FALSE
)

grid.rect(1:2/3, width=1/4, height=1/2,
          gp=gpar(fill=list(grad, pat)))

^{Created on 2022-10-30 by the reprex package (v2.0.1)}

Let's say we might want to try to adopt this in ggplot2, like so:

library(ggplot2)

ggplot(data.frame(x = 1:2), aes(x, x)) +
  geom_tile(aes(fill = factor(x)), width = 1) +
  scale_fill_manual(
    values = list(grad, pat)
  )
#> Error in `geom_tile()`:
#> ! Problem while converting geom to grob.
#> ℹ Error occurred in the 1st layer.
#> Caused by error:
#> ! Unknown colour name: list(x1 = 0, y1 = 0.5, x2 = 1, y2 = 0.5, stops = c(0, 1), colours = c("#DF536B", "white"), extend = "pad", group = FALSE)

As can be seen above, we currently can't use this in ggplot2. I think the only thing currently prohibiting this is the alpha() function that can't handle a list of patterns as input.

If I change the alpha() function to be like this (real solution should probably be more elegant):

alpha <- function(colour, alpha = NA) {
  
  if (is.list(colour)) {
    return(colour)
  }
  
  ... # rest of function body
}

And build the package and re-load ggplot2 (which doesn't lend itself well to reprexes), I can get exactly what I might have expected:

library(ggplot2)

ggplot(data.frame(x = 1:2), aes(x, x)) +
  geom_tile(aes(fill = factor(x)), width = 1) +
  scale_fill_manual(
    values = list(grad, pat)
  )

[thomasp85]

Is this still needed in your current ggplot2 PR for pattern fills?

[teunbrand]

No, this issue should be fixed by tidyverse/ggplot2#5299