title: Add a legend to the ggmap
body: <p>I am trying to add a legend to a <code>ggmap</code> like <a href="https://stackoverflow.com/questions/10918102/r-adding-legend-to-ggmap-ggplot2-while-using-annotate?rq=1">here</a>. It looks very good. But the first problem is, that <code>scale_colour_continuous</code> is only for two colour, but I need to use three colours. Secondly, event when I tried to use only two colors (which looked very bad), I failed. My code looks like:</p>
  
  <pre><code> ee&lt;-runif(100,min=-6, max=26)
   nn&lt;-runif(100, min=30, max=75)
   r&lt;-colorRampPalette(colors=c("white", "red2", "black"))
   aa&lt;-round(runif(100, min=1, max=35),0)
   barv&lt;-r(10)[as.numeric(cut(aa, breaks=10))]
   z&lt;-data.frame(ee, nn,barv)
   my_map&lt;-get_map(location='Europe', zoom = 3, maptype='satellite')
   p2&lt;-ggmap(my_map)+
   theme(text=element_text(size=15))
   p2&lt;-p2+geom_point(data=z, aes(x=ee, y=nn), colour=barv, 
   alpha = 0.5, size=2.5)
   p2
   #p2 + scale_color_continuous(low="white", high="black", 
   #space = "Lab", guide = "colorbar")
  </code></pre>
  
  <p>Can anyone help please?</p>