title: ggplot2 - Change `geom_rect` colour in a stacked barplot
body: <p>I am trying to plot a stacked barplot using <code>ggplot2::geom_bar</code> with backgroud shading (using <code>ggplot2::geom_rect()</code>) according to a categorical variable as follows:</p>
  
  <pre><code>shading &lt;- data.frame(min = seq(from = 0.5, to = max(as.numeric(as.factor(diamonds$clarity))), by = 1),
                        max = seq(from = 1.5, to = max(as.numeric(as.factor(diamonds$clarity))) + 0.5, by = 1),
                        col = c(0,1))
  
  ggplot() + 
    theme(panel.background = element_rect(fill = "transparent")) +
    geom_bar(data = diamonds, mapping = aes(clarity, fill=cut)) +
    geom_rect(data = shading,
              aes(xmin = min, xmax = max, ymin = -Inf, ymax = Inf,
                  fill = factor(col), alpha = 0.1)) +
    geom_bar(data = diamonds, mapping = aes(clarity, fill=cut)) +
    guides(alpha = FALSE)
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/pw2gH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/pw2gH.png" alt="enter image description here"></a></p>
  
  <p>How to change the colours of the shading? 
  I have tried <code>scale_fill_manual(values = c("white", "gray53"))</code>, but it seems that multiple scale aesthetics are not possible in <code>ggplot2</code> (<a href="https://github.com/hadley/ggplot2/issues/578" rel="noreferrer">https://github.com/hadley/ggplot2/issues/578</a>). Is there another way to get the desired result ?</p>