title: geom_abline legend - How can I add abline text legend
body: <p>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.</p>
  
  <p>Can you help me?</p>
  
  <pre><code>library(ggplot2)
  
  amostra &lt;- 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))
  </code></pre>