title: How to set a constant "ylim" in geom_raster?
body: <p>I have a kind of "time series", with different measures taken at regular points on the same individuals.
  I want to graphically represent 2 of these time series on the same graph (no problem with that), and add a background which depends on a third factor.</p>
  
  <p>Here a reproducible example of what I've done: </p>
  
  <pre><code>df &lt;- data.frame(
    x = seq(1, 20),
    y = sample(c(1:10), 20, replace = TRUE),
    z = sample(c(1:10), 20, replace = TRUE),
    w = sample(c("yes", "no"), 20, replace = TRUE)
  )
  
  ggplot(df) +
    geom_line(aes(x = x, y = y), color = 'darkorange') +
    geom_line(aes(x = x, y = z), color = 'royalblue') +
    geom_raster(aes(x = x, y = 5, fill = w, alpha = w)) +
    scale_alpha_ordinal(range = c(0, 0.8)) +
    scale_fill_manual(values = c("gray32", "gray32"))
  </code></pre>
  
  <p>Which give me almost what I want excepted that I would like my raster to cover my whole y-axis window.</p>
  
  <p>Any idea?</p>
  
  <p>Thank you!</p>