ggplot2 - How to add legend to ggplot manually? - R

Question

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Answer 1

ggplot really only likes to draw legends for things that have aesthetic mappings. If you set "code names" for colors, you can define them in a manual scale for that attribute. For example

ggplot(df, aes(x = instance, y = total_hits)) +
  geom_point(size = 1) + 
  geom_line()+
  geom_line(aes(x=instance, y = line1, colour="myline1")) +
  geom_vline(xintercept=805) + 
  geom_line(aes(x=instance, y = line2, colour="myline2"))+
  geom_line(aes(x=instance, y = line3, colour="myline3")) +
  scale_colour_manual(name="Line Color",
      values=c(myline1="red", myline2="blue", myline3="purple"))

should work (untested since you didn't provide any data at all). Anytime you ask a question, it's just polite to include a reproducible example so the answer-er doesn't have to do all the work themselves to test.