Additional legend in ggplot2

Question

I have a problem where the legend of my ggplot() does not appear. Here\'s my code:

plot_bt <- ggplot(NULL, aes(x, v1)) + 
  geom_line(data = nig_         


        

Answer 1

Just do this:

plot_bt <- ggplot(NULL, aes(x, v1)) + 
  geom_line(data = nig_bt_1, aes(colour = "a")) +
  geom_line(data = nig_bt_2, aes(colour = "b")) +
  geom_line(data = nig_bt_3, aes(colour = "c")) + 
  labs(x = "X", y = "Probability") +
  scales_color_manual(values= c("a" = "black", "b" = "blue", "c" = "red"))

A guide can only depict mappings you've defined using aes. The ggplot2 way is of course to first combine the data and use a grouping variable.