Adding legend to ggplot2 with multiple lines on pl

Question

I would really appreciate some help with this. I completely do not understand how ggplot2 is thinking about legends!

The chart is based on the following data frame, called "meltdf":

        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

The chart is drawn as follows:

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))

I would like to have a legend at the bottom in which all three lines in the chart (blue, red and grey) are named.

Answer 1

ggplot will automatically produce legend for the elements (colours, linetype, etc.) that are mapped inside an aes() call. It means that you will not get a legend when you manually specify the colour of an element, like you do here (all your colour statements are not inside aes).

I recommend you store the coordinates of your vertical lines in their own dataframe, where they can be mapped to a variable giving their color. Here, I create such a dataframe (summ) and rewrote your code for geom_vline(). I also mapped meltdf$variable to colour in geom_vline so it appears in the legend. Finally, I added scale_colour_manual() to choose the colours. You may want to adapt the value of summ$colour so they make sense in the legend.

summ <- data.frame(x=c(22,30),
                   colour=c("gray","red"))

ggplot(data=meltdf,aes(x=xval,y=value))+
  geom_line(size=1,aes(colour=variable))+
  geom_vline(data=summ,aes(xintercept = x,colour=colour))+
  scale_color_manual(values = c(rgb(69,99,111,max=255),"darkgray","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))