title: Adding legend to ggplot2 with multiple lines on plot
body: <p>I would really appreciate some help with this. I completely do not understand how ggplot2 is thinking about legends!</p>
  
  <p>The chart is based on the following data frame, called "meltdf":</p>
  
  <pre><code>        xval       variable  value
  1   0.000000 Shortfall Risk 100.00
  2   4.624457 Shortfall Risk  99.83
  3   9.179656 Shortfall Risk  60.96
  4  13.742579 Shortfall Risk  36.29
  5  18.620338 Shortfall Risk  27.71
  6  22.947704 Shortfall Risk  22.52
  7  27.690638 Shortfall Risk  19.72
  8  32.174379 Shortfall Risk  17.89
  9  36.637940 Shortfall Risk  15.79
  10 41.107962 Shortfall Risk  15.96
  11 45.644065 Shortfall Risk  15.97
  </code></pre>
  
  <p>The chart is drawn as follows:</p>
  
  <pre><code>ggplot(data=meltdf,aes(x=xval,y=value))+
    geom_line(size=1,colour=rgb(69,99,111,max=255))+
    geom_vline(xintercept = 22 ,colour="darkgray")+
    geom_vline(xintercept = 30 ,colour="darkred")+
    theme_bw()+
    labs(title="Shortfall Risk versus Investment Risk, Meeting Expenditure Only")+
    theme(legend.position="bottom",
      legend.text = element_text(size=9),
      axis.text = element_text(size=9),
      axis.title = element_text(size=9),
      plot.title=element_text(size = 9),
      legend.title=element_text(size=9))+
    labs(x="Largest Historical Decline (%)", y="Probability of Shortfall (%)")+
    scale_y_continuous(limits = c(0, 100))
  </code></pre>
  
  <p>I would like to have a legend at the bottom in which all three lines in the chart (blue, red and grey) are named. </p>