title: ggplot2 tile scale limits
body: <p>Apologies if this is a bit of an obvious question, but can anyone tell me how I can set the scale on a heatmap drawn with geom tile? I want to draw several heatmaps all on the same scale (e.g. drawn with the colours scaled to  a maximum of 10 even though on two of them the maximum is 5) and can't work it out.</p>
  
  <p>An example:</p>
  
  <pre><code>mydata&lt;-expand.grid(X1=1:8,Y1=1:5)
  Z1&lt;-floor(runif(40,min=0,max=6))
  Z2&lt;-floor(runif(40,min=0,max=11))
  
  mydata&lt;-cbind(mydata,Z1,Z2)
  
  mydata&lt;-data.frame(mydata)
  
  p1 &lt;- ggplot(mydata, aes(x=X1, y=Y1, z = Z1))
  p1 &lt;- p +geom_tile(aes(fill = Z1))
  
  p2 &lt;- ggplot(mydata, aes(x=X1, y=Y1, z = Z2))
  p2 &lt;- p +geom_tile(aes(fill = Z2))
  </code></pre>
  
  <p>p1 will be scaled between 0 and 4 whereas p2 will be scaled between 0 and 10. I'd like to draw both of them so that the colours are on the same scale - so "4" has the same colour in both heatmaps.</p>
  
  <p>Thanks for any help</p>