title: Plot background colour in gradient
body: <p>This code produces the first plot below:</p>
  
  <pre><code>water.height &lt;- seq(0, 5, 1)
  y &lt;- seq(0, 1500, length.out = 6)
  df &lt;- data.frame(water.height, y)
  
  library(ggplot2)
  ggplot(df, aes(water.height, y)) + geom_blank()+ theme_bw()
  </code></pre>
  
  <p><img src="https://i.stack.imgur.com/MqZLa.jpg" alt="enter image description here"></p>
  
  <p>I have photoshopped in this blue background:</p>
  
  <p><img src="https://i.stack.imgur.com/9fWHy.jpg" alt="enter image description here"></p>
  
  <p>Can I produce the same blue background with R code?</p>