title: Legend linetypes not displaying properly ggplot
body: <p>I have the following plot: </p>
  
  <pre><code>ggplot(proba[108:140,], aes(c,four, color="a1")) + 
     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, color="a2"), size=0.2, linetype=2) + 
     geom_line(aes(c,one, color="a3"),linetype=3, size=0.8) + 
     geom_line(aes(c,two, color="a4"), linetype=1, size=0.6) + 
     scale_color_manual(values=c("a1"="red3", "a2"="red3","a3"="blue3","a4"="blue3")) + 
     theme(legend.title = element_text(colour="black", size=7)) + 
     theme(legend.position="bottom" ,
           legend.direction="horizontal", 
           legend.title=theme_blank()) + 
     theme(legend.text=theme_text(size=7), 
           legend.background=theme_blank(), 
           legend.key=theme_blank())
  </code></pre>
  
  <p>The four lines on the plot are displayed in different linetypes, however the legend does not show these different linetypes only the different colours. I am obviously missing a very simple argument in theme() but I cannot figure out what it is. Maybe I need to specify the linetypes again in scale_colour_manual()? </p>
  
  <p>Here's the data: <a href="https://dl.dropboxusercontent.com/u/22681355/proba.csv" rel="nofollow">https://dl.dropboxusercontent.com/u/22681355/proba.csv</a></p>
  
  <pre><code>proba&lt;-read.csv("proba.csv",head=T)
  </code></pre>