title: R - ggplot2 - use raster as greyscale basemap
body: <p>I have two raster layers, one is a greyscale image and the other results from an analysis. I'd like to use one raster as greyscale background in ggplot and superimpose the second raster using a color-ramp. This is an example how it could look like (but nicer):</p>
  
  <p><a href="https://i.stack.imgur.com/jHNDF.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/jHNDF.png" alt="enter image description here"></a></p>
  
  <p>I can't assign separate colors to the rasters. Do you know a way to achieve this? I googled for a long time and I have the impression that this is not possible. But I can't (and don't want to) belive it.</p>
  
  <p>Please let me know if you need further infomrmation.</p>
  
  <p><strong>Example</strong></p>
  
  <p>create test data</p>
  
  <pre><code>xy.1 &lt;- expand.grid(1:10, 1:10)
  df.1 &lt;- data.frame(Longitude=xy.1[,1], Latitude=xy.1[,2], Value=xy.1[,1]+xy.1[,2])
  xy.2 &lt;- expand.grid(3:5, 3:5)
  df.2 &lt;- data.frame(Longitude=xy.2[,1], Latitude=xy.2[,2], Value=rnorm(9))
  </code></pre>
  
  <p>plot</p>
  
  <pre><code>p1 &lt;- ggplot() +
    geom_raster(data=df.1, aes(x=Longitude, y=Latitude, fill=Value)) +
    scale_fill_gradientn(colours = grey(seq(0,1,l=20))) +
    coord_equal() +
    theme(legend.position="none")
  
  p1 + geom_raster(data=df.2, aes(x=Longitude, y=Latitude, fill=Value)) +
    scale_fill_gradient(low="red", high="white")
  </code></pre>
  
  <p>Result:</p>
  
  <p><a href="https://i.stack.imgur.com/05OXW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/05OXW.png" alt="enter image description here"></a></p>