texture in geom_polygon fill - r

Question

I need to create a European map to show the distribution of a variable across countries. I need the map in black and white. I rely on ggplot and followed this approach as an example. I changed the legend based on this blogpost. All this works fine with this result:
My question is how to change the map in a way that the countries where I am missing the information for fill and are shown as pure white have a texture over-them (I am thinking diagonal lines)?
Since my script is a bit messy, I just show the ggplot here, without the data preparation part:
require(ggplot2)
plotCoords <- read.csv("http://eborbath.github.io/stackoverflow/PlotCoords.csv")
showCoords <- read.csv("http://eborbath.github.io/stackoverflow/showCoords.csv")
ggplot() +
geom_polygon(
data = plotCoords,
aes(x = long, y = lat, group = group),
fill = "white", colour = "darkgrey", size = 0.6) +
geom_polygon(
data = showCoords,
aes(x = long, y = lat, group = group),
fill = "grey", colour = "black", size = 0.6) +
geom_polygon(
data = showCoords,
aes(x = long, y = lat, group = group, fill = sh_left),
colour = "black", size = 0.1) +
scale_fill_gradient(
low = "gray90", high = "gray0",
name = "Share of left-wing protesters",
guide = guide_colorbar(
direction = "horizontal",
barheight = unit(2, units = "mm"),
barwidth = unit(50, units = "mm"),
draw.ulim = F,
title.position = 'top',
title.hjust = 0.5,
label.hjust = 0.5
)) +
scale_x_continuous(element_blank(), breaks = NULL) +
scale_y_continuous(element_blank(), breaks = NULL) +
coord_map(xlim = c(-26, 47), ylim = c(32.5, 73)) +
theme_bw() +
theme(legend.justification = c(-0.4, 1.2), legend.position = c(0, 1))
The first geom_polygon is for the background, I assume I have to edit the fill there. Obviously, this is important to differentiate no information from low values of the variable I plot. Given I have to rely on black and white I came up with the idea of using textures, but I am open to alternative suggestions.
Thanks!

Answer 1

it's technically possible with gridSVG, but not sure it's worth the effort.
I created a new geom based on GeomPolygon, and modified the draw_panel method to return,
gl <- by(munched, munched$group,
function(m){
g <- polygonGrob(m$x, m$y, default.units = "native")
patternFillGrob(g,
pattern = pattern(linesGrob(gp=gpar(col="red",lwd=3)),
width = unit(2, "mm"), height = unit(2, "mm"),
dev.width = 1, dev.height = 1))
}, simplify = FALSE)
gTree(children = do.call(gList, gl))