title: Fill or colour in continuous scale through gradient of more than 2 defined colours
body: <p>Is it possible to make a colour gradient through more than low and high defined parameters?</p>
  
  <p>Lets say, on this data:</p>
  
  <pre><code>df &lt;- data.frame(a = 1:100, 
                   b = rnorm(100, mean = 1000, sd = 500))
  ggplot(df) +
     geom_point(aes(a, b)) +
     scale_fill_continous(low = "white", high = "black")
           ## as "low" and "high" is all scale_fill_continous() funcion offer
  </code></pre>
  
  <p>Setting the low and high parameters only results in a linear colour gradient, but I want more combined colour gradient, lets say from white to blue and then to black.</p>
  
  <p>Thank you for your attention and answers.</p>