title: geom_tile different gradient scale and color for different factors
body: <p>Hi I would like to plot the following dataframe. </p>
  
  <pre><code>d&lt;- data.frame (pid=c("d","b","c"), type=c("rna","rna","rna"), value = c(1,2,3) )
  d2 &lt;- data.frame (pid=c("d","b","c"), type=c("dna","dna","dna"), value = c(10,20,30) )
  df &lt;- rbind (d,d2)
  
   ggplot(df, aes(y=pid, x=type  ) ) + geom_tile(aes(fill = value),
    colour = "white") + scale_fill_gradient(low = "white",
    high = "steelblue") 
  </code></pre>
  
  <p>This produces a plot that looks like this, 
  <a href="https://i.stack.imgur.com/jXhpu.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jXhpu.png" alt="enter image description here"></a></p>
  
  <p>however, I would like to have each x factor have its own color gradient, so ideally rna is blue to white while dna is red to white.  Is there anyway to do this? Of if different gradient is not possible, then what about just different scales? thanks! </p>