title: Add shaded region across all panels in ridgeline plot
body: <p>I have a dataframe <code>df</code>: </p>
  
  <pre><code> set.seed(3467)
  
   df&lt;- data.frame(method= c(rep("A", 1000), rep("B", 1000), rep("C", 1000)), 
              beta=c(rnorm(1000, mean=0, sd=1),rnorm(1000, mean=2, sd=1.4),rnorm(1000, mean=0, sd=0.5)))
  </code></pre>
  
  <p>I wish to create a ridge plot like so: </p>
  
  <pre><code>ggplot(df, aes(x = beta, y = method, color = method, fill = method)) +
  geom_density_ridges(size=1)+
  scale_fill_manual(values = c("#228b2250", "#0072B250", "#483d8b50")) +
  scale_color_manual(values = c("#228b22", "#0072B2", "#483d8b"), guide = "none") +
  stat_density_ridges(quantile_lines = TRUE, quantiles = c(0.025, 0.5, 0.975), alpha = 0.6, size=1)
  </code></pre>
  
  <p>I wish to add a shaded rectangle like so:</p>
  
  <pre><code>ggplot(df) +
  geom_rect(data = data.frame(x = 1),
          xmin = -1, xmax = 1, ymin = -Inf, ymax = Inf,
          alpha = 0.5, fill = "gray") +
  geom_density_ridges2(aes(x = beta, fill = method, y = method))
  </code></pre>
  
  <p>However, this attempt is unsuccessful: </p>
  
  <pre><code>ggplot(df, aes(x = beta, y = method, color = method, fill = method)) +
  geom_density_ridges(size=1)+
  scale_fill_manual(values = c("#228b2250", "#0072B250", "#483d8b50")) +
  scale_color_manual(values = c("#228b22", "#0072B2", "#483d8b"), guide = "none") +
  stat_density_ridges(quantile_lines = TRUE, quantiles = c(0.025, 0.5, 0.975), alpha = 0.6, size=1)+
  geom_rect(data = data.frame(x = 1),
          xmin = -1, xmax = 1, ymin = -Inf, ymax = Inf,
          alpha = 0.5, fill = "grey") 
  </code></pre>
  
  <p>The <code>geom_rect()</code> function does not seem to work with <code>geom_density_ridges()</code> only <code>geom_density_ridges2()</code>. </p>