title: Add abline to legend
body: <p>First, sorry for posting without reproducible data. Hope you guys understand my question. This is my code. At the end of the code, I am trying to add abline. With the code, I am trying to add the name of abline to the legend but it does not work.<a href="https://i.stack.imgur.com/cKR6h.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/cKR6h.png" alt="enter image description here"></a></p>
  
  <pre><code>ggplot(aes(x = week_id2, y = Index, color = Chain2, linetype = Chain2, group = Chain2), 
         data = data00 +
    geom_point(aes(shape=Chain2), size = 3) +  
    geom_line() + 
    scale_linetype_manual(values=c("twodash", "dashed", "dotted", "dotdash", "longdash")) + 
    scale_shape_manual(values=c(1:5)) +
    xlab("Week") + 
    ylab("Index") + 
    geom_hline(aes(yintercept=1)) 
  </code></pre>
  
  <p>As shown, I just simply add a name of the abline (let's say the name is "add") in the legend.  How should I do it with my current code?</p>