Adding legend to ggplot2 with multiple lines on plot

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, call

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))