title: How to overlay two heatmaps via ggplot2 with two different scales_fill_gradient?
body: <p>I have a data which has two variables and I want to see a single plot with heatmap for each of them overlaid on one another and showing two color scales for the two different variables. My code while not correct should clearly indicate what I am trying to achieve. </p>
  
  <p>I have looked through several examples none of those indicate how to do this for geom_tile(). It would have been easy for geom_point. I am providing a synthetic example to show what I am doing. I get the error saying "Scale for 'fill' is already present. Adding another scale for 'fill', which will replace the existing scale." Evidently it is accepting only the second scale_fill_gradient, but I would like to view both the color gradients corresponding to the variables in the same heatmap. 
  It would be great if I could find a way to get this plot. Thank you!</p>
  
  <pre><code>library(reshape2)
  library(ggplot2)
  
  set.seed(2)
  m1 = matrix(rnorm(100), nrow=10)
  m2 = matrix(rnorm(100), nrow=10)
  M1 = melt(m1)
  M2 = melt(m2)
  names(M1)  = c("Var1", "Var2", "value1")
  names(M2)  = c("Var1", "Var2", "value2")
  pp1 &lt;- ggplot() +
    geom_tile(data=M1, aes(x=Var1, y=Var2, fill=value1)) +
    scale_fill_gradient(low="white", high="red") +
    geom_tile(data=M2, aes(x=Var1, y=Var2, fill=value2)) +
    scale_fill_gradient(low="blue", high="yellow")
  pp1
  </code></pre>