feat: Use 'pattern_fill2' to fill in the color of the top 'crosshatch' hatch

[trevorld]

• One possible enhancement would be to use the 'pattern_fill2' aesthetic to color in the top 'crosshatch' pattern "hatch"
• By default use the the 'pattern_fill' value (which would be reverse-compatible)
• This would require fairly modest changes to your underlying grid grob code
• This would allow a third 'crosshatch' pattern be the combination of two 'stripe' patterns (for use in maps of spoken language distributions or Venn diagrams)? Here is toy example using a custom geometry-pattern that implements this:

library("ggpattern")
library("ggplot2", warn.conflicts = FALSE)
library("gridpattern") # remotes::install_github("trevorld/gridpattern")

crosshatch2_pattern <- function(params, boundary_df, aspect_ratio, legend) {
    args <- as.list(params)
    args <- args[grep("^pattern_", names(args))]

    args$pattern <- "crosshatch"
    args$x <- boundary_df$x
    args$y <- boundary_df$y
    args$id <- boundary_df$id
    args$prefix <- ""

    do.call(gridpattern::patternGrob, args)
}

options(ggpattern_geometry_funcs = list(crosshatch2 = crosshatch2_pattern))
df <- data.frame(trt = c("a", "b", "c"), outcome = c(2.3, 1.9, 3.2),
                 pattern = c("stripe", "stripe", "crosshatch2"),
                 pattern_angle = c(30, -60, 30),
                 pattern_fill = c("grey40", "white", "grey40"),
                 pattern_fill2 = c(NA, NA, "white"))
ggplot(df, aes(trt, outcome)) +
    geom_col_pattern(aes(fill = trt),
                     pattern = df$pattern,
                     pattern_angle = df$pattern_angle,
                     pattern_fill = df$pattern_fill,
                     pattern_fill2 = df$pattern_fill2,
                     colour = 'black')