title: Bar graph and geom_line in ggplot
body: <p>I am trying to draw a geom_line on a bar chart, my bars are filled by year.
  My code is:</p>
  
  <pre><code>library(ggplot2)
  library(plyr)
  library(reshape)
  
  DF &lt;- data.frame(DECL.INDICATORS=c("Finland", "Finland", "Germany" ,"Germany","Italy","Italy"),
                   Year=c(2009,2010,2009,2010,2009,2010),
                   EXPVAL=c(2136410,1462620,371845300,402397520,357341970,357341970),
                   IMPVAL=c(-33668520,-37837140,-283300110,-306157870,-103628920,-105191850))
  
  
  net &lt;- ddply(DF, .(Year,DECL.INDICATORS), summarise, 
                  net = sum(EXPVAL + IMPVAL))
  
  DF.m &lt;- melt(DF, id.vars = c("Year", "DECL.INDICATORS"))
  
  ggplot(DF.m,aes(x=DECL.INDICATORS,y=value, fill=factor(Year)))+
    geom_bar(stat="identity",position="dodge",colour="darkgreen")
  
  last_plot() + geom_line(data = net, aes(DECL.INDICATORS, net,group = 1), size = 1) + geom_hline(yintercept = 0,colour = "grey90")
  </code></pre>
  
  <p>Problem I am trying to resolve is to draw a three lines (net export from <code>net</code>) for each country Finland, Germany, Italy.</p>
  
  <p>With my last code line  i am getting only three point which are connected with lines</p>