title: Add raster to ggmap base map: set alpha (transparency) and fill color to inset_raster() in ggplot2
body: <p>I want to plot a map with a raster overlaying a GoogleMaps base map in ggplot2. Therefore, I used <code>get_map()</code> and <code>insert_raster()</code> like this:</p>
  
  <pre><code>library(ggplot2)
  library(ggmap)
  
  bm &lt;- ggmap(get_map(location = "Bangkok", maptype = "hybrid"))
  
  bm + inset_raster(as.raster(r), xmin = r@extent[1], xmax = r@extent[2],
                    ymin = r@extent[3], ymax = r@extent[4])
  </code></pre>
  
  <p>Is there any possibility to set a <code>alpha</code> and change the <code>fill</code> color?</p>
  
  <p>The result looks like this:
  <a href="https://i.stack.imgur.com/1V0QS.png" rel="noreferrer"><img src="https://i.stack.imgur.com/1V0QS.png" alt="enter image description here"></a></p>