title: Adding legend to ggplot when lines were added manually
body: <p>I have a plot in ggplot with 4 separate lines that I have added with a separate geom_line() argument. I would like to add legend but scale_colour_manual doesn't work in this case. What is the proper way to add legends when I added the variables separately?</p>
  
  <p>Here's my code:</p>
  
  <pre><code>ggplot(proba[108:140,], aes(c,four)) + 
      geom_line(linetype=1, size=0.3) + 
      scale_x_continuous(breaks=seq(110,140,5)) + 
      theme_bw() + 
      theme(axis.line = element_line(colour = "black", size=0.25),
            panel.grid.major = element_blank(),
            panel.grid.minor = element_blank(),
            panel.border = element_blank(),
            panel.background = element_blank()) + 
      theme(axis.text.x = element_text(angle = 0, hjust = +0.5, size=6,color="black")) + 
      theme(axis.text.y = element_text(angle = 0, hjust = -100, size=6, color="black")) + 
      theme(axis.ticks=element_line(colour="black",size=0.25)) + 
      xlab("\nTime-steps") + 
      ylab("Proportion correct\n") + 
      theme(axis.text=element_text(size=8),axis.title=element_text(size=8)) + 
      geom_line(aes(c,three), size=0.2, linetype=2) + 
      geom_line(aes(c,one),linetype=3, size=0.8, colour="darkgrey") + 
      geom_line(aes(c,two), linetype=1, size=0.8, colour="darkgrey")
  </code></pre>