title: Setting values for scale_colour_gradientn in ggplot2
body: <p>I'm trying to use scale_colour_gradientn to clearly show neg vs pos values: negative is blue, positive is red.  But gradientn seems to ignore my vector of colors, and just makes one big gradient instead of n small gradients.  Here's my sample code ...</p>
  
  <pre><code>xx &lt;- -100:100
  yy &lt;- xx
  fma &lt;- data.frame( xx, yy)
  ggplot( fma) +
    geom_point( aes( xx, yy, col=yy)) +
    scale_colour_gradientn(
      colours=c('#0000ff','#0000ff','#ff0000','#ff0000'),
      values=c(  -100,       -1,      1,      100))
  </code></pre>
  
  <p>How can I convince gradientn to color everything in [-100,-1] blue and everything in [1,100] red?</p>