title: How to set fixed continuous colour values in ggplot2
body: <p>I'm plotting a lot of graphics and I'd like for all of them to have the same colour scale so I can compare one to another. Here's my code:</p>
  
  <pre><code>myPalette &lt;- colorRampPalette(rev(brewer.pal(11, "Spectral")))
  print(ggplot(mydata, aes(x= X, y= Y, colour= Z)) + geom_point(alpha=.5,size = 6) + scale_colour_gradientn(colours = myPalette(100)) + ylim(.1,.4) + xlim(1.5,2) + ggtitle(title))
  </code></pre>
  
  <p>Is there a way to set this colour scale?</p>