title: Add legend to geom_line() graph in r
body: <p>I've been trying to add legend to my ggplot, but failed miserably.  I tried the function <code>scale_colour_manual()</code>, but the legend doesn't show up. </p>
  
  <pre><code>ggplot()+
  geom_line(data=Summary,aes(y=Y1,x= X),colour="darkblue",size=1 )+
  geom_line(data=Summary,aes(y=Y2,x= X),colour="red",size=1  )
  </code></pre>
  
  <p>My dataframe 'Summary' is as follows:</p>
  
  <pre><code>  X           Y1           Y2
  139 1.465477e+16 7.173075e+15
  277 1.044803e+16 9.275002e+15
  415 1.059258e+16 8.562518e+15
  553 1.033283e+16 8.268984e+15
  691 9.548019e+15 1.022248e+16
  830 1.008212e+16 8.641891e+15
  968 9.822061e+15 9.315856e+15
  1106 9.948143e+15 9.178694e+15
  1244 1.013922e+16 8.825904e+15
  1382 9.815094e+15 9.283662e+15
  </code></pre>
  
  <p>Please advise me how to plot Y1, Y2 against X on the same graph and add a legend on the side.</p>