geom_abline legend - How can I add abline text legend

Question

I'm trying to build charts control. I want to explicit that green line (abline1) is central line and red line (abline2 and abline3) are the limits control.

Can you help me?

library(ggplot2)

amostra <- rnorm(30, 10)

qplot(y = amostra, x = seq_along(amostra), ylim = c(5,15), main = "Gráfico de Controle", xlab = 'Período', ylab = 'Valores') + geom_line() +
geom_abline(aes(slope=0, intercept=10), colour = 'green', size=1) +
geom_abline(aes(slope=0, intercept=7), colour='red', size = 1) +
geom_abline(aes(slope=0, intercept=13), colour='red', size = 1) +
theme(plot.title = element_text(hjust = 0.5))

Answer 1

If you want a legend, then you need an aesthetic mapping. Move color in the aes() and add a scale_color_identity() . You can do this with

qplot(y = amostra, x = seq_along(amostra), ylim = c(5,15), main = "Gráfico de Controle", xlab = 'Período', ylab = 'Valores') + geom_line() +
  geom_abline(aes(slope=0, intercept=10, colour = 'green'), size=1) +
  geom_abline(aes(slope=0, intercept=7, colour='red'), size = 1) +
  geom_abline(aes(slope=0, intercept=13, colour='red'), size = 1) +
  theme(plot.title = element_text(hjust = 0.5)) + 
  scale_color_identity(labels=c("central","limits"), guide="legend")

Answer 2

This is a rather bad hack, but to force the lines to be in the legend, you could try to draw them using the geom_errorbarh() function. This will plot errorbars. If you make them as wide as the plot while setting the plot width explicitly with scale_x_continuous() , you can have horizontal bars that are shown in figure legend.