title: Filling density plot with colours
body: <p>I have this <code>ggplot2</code> script:</p>
  
  <pre><code>require(reshape2)
  library(ggplot2)
  library(RColorBrewer)
  library(extrafont)
  font_import(pattern = 'Arch') 
  font_import(pattern = 'Akk') 
  
  fileName = paste("/ex_spectra.csv", sep = "") # data: https://www.dropbox.com/s/gs1hwv3xjnlhb7z/ex_spectra.csv?dl=0
  mydata = read.csv(fileName,sep=",", header=TRUE, check.names=FALSE)
  dataM = melt(mydata,c("x"))
  
  my_palette = c(brewer.pal(5, "Set1")[c(1,2,3,4,5)])
  
  ggplot(data=dataM, aes(x= x, y=value, colour=variable, fill = variable, size = variable)) +
  geom_line(alpha = .75) +
  
  scale_colour_manual(breaks=c("A", "B", "C", "D", "E"), values=my_palette) +
  scale_size_manual(breaks=c("A", "B", "C", "D", "E"), values=c(0.7,0.7,0.7,0.7,0.7)) +
  
  theme(plot.background = element_blank(), panel.grid.minor = element_blank(), #panel.grid.major = element_blank(),
  axis.line = element_blank(),
  legend.key = element_blank(), legend.title = element_blank()) +
  scale_y_continuous("y", expand=c(0,0)) + 
  scale_x_continuous("x", expand=c(0,0)) +
  
  theme(axis.title.x = element_text(vjust=-0.3, face="bold", size=12, colour = "grey50", family="AkkuratPro-Regular")) + 
  
  theme(axis.title.y = element_text(vjust=1.5, face="bold", size=12, colour = "grey50", family="AkkuratPro-Regular")) +
  
  theme(legend.position = "right", axis.ticks = element_blank(),
    axis.text.x = element_text(size = 9, angle = 0, vjust = 0.25 , hjust = 1, colour = "grey50", family="AkkuratLightPro-Regular")
    ,axis.text.y = element_text(size = 9, angle = 0, hjust = 1, colour = "grey50", family="AkkuratLightPro-Regular"))
  </code></pre>
  
  <p>Which produces this plot:</p>
  
  <p><img src="https://i.stack.imgur.com/vVMbo.png" alt="enter image description here"></p>
  
  <p>Now, I'd like to fill the density curves with their corresponding colours (at, say, 25% alpha). Isn't <code>fill = variable</code> the usual way to do this?</p>