title: Create a symmetric colour scale with scale_colour_gradient(low="red", high="blue") in ggplot2
body: <pre><code>scale_colour_gradient(low="red", high="blue")
  </code></pre>
  
  <p>generates a colour gradient that looks mostly red. I expected <code>red-violet-blue</code> with violet at in the middle but I got <code>red-still_red-violet-blue</code></p>
  
  <p><img src="https://i.stack.imgur.com/dqGF2.png" alt="red-still_red-violet-blue"></p>
  
  <p>How can I get a gradient with violet centered?</p>