title: texture in geom_polygon fill
body: <p>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 <code>ggplot</code> and followed this <a href="https://github.com/brechtdv/ggplot2-EU-map" rel="nofollow noreferrer">approach</a> as an example. I changed the legend based on <a href="https://timogrossenbacher.ch/2016/12/beautiful-thematic-maps-with-ggplot2-only/" rel="nofollow noreferrer">this blogpost</a>. All this works fine with this result: <a href="https://i.stack.imgur.com/kn0nv.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/kn0nv.png" alt="enter image description here"></a></p>
  
  <p>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)? </p>
  
  <p>Since my script is a bit messy, I just show the ggplot here, without the data preparation part:</p>
  
  <pre><code>require(ggplot2)
  
  plotCoords &lt;- read.csv("http://eborbath.github.io/stackoverflow/PlotCoords.csv")
  showCoords &lt;- 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))
  </code></pre>
  
  <p>The first <code>geom_polygon</code> is for the background, I assume I have to edit the <code>fill</code> 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.</p>
  
  <p>Thanks!</p>