title: ggplot2: Legend for labeling multiple geom_abline slopes
body: <p>I am using this <code>data.frame</code>, which I have <code>dput</code> below, to map three slopes using <code>geom_abline</code>:</p>
  
  <pre><code>  moderator_value simple_intercept simple_slope
  1              -1      -0.02745523    0.2768973
  2               0       0.05990693    0.2147829
  3               1       0.14726909    0.1526684
  </code></pre>
  
  <p>What I have right now is this code:</p>
  
  <pre><code>ggplot() +
    geom_abline(data=ablines, 
                mapping=aes(slope=simple_slope, intercept=simple_intercept),
                linetype=c(1,2,3)) +
    scale_x_continuous(limits=c(-1.5,2), name="Prejudice") +
    scale_y_continuous(limits=c(-.75, .75), name="Authenticity") +
    theme_light() +
    theme(text=element_text(size=14))
  </code></pre>
  
  <p>This returns the figure:</p>
  
  <p><a href="https://i.stack.imgur.com/4kVhW.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/4kVhW.png" alt="enter image description here"></a></p>
  
  <p>I would like to add a legend that labels these three separate lines by their <code>linetype</code>. I have looked elsewhere on SO, but many of these say just to include <code>show_guide</code> in the <code>geom_abline</code> function (which has now been deprecated in favor of <code>show.legend</code>) and set it to <code>TRUE</code>. This is not working for me. I've tried to use <code>scale_linetype_manual</code>, as well, with no luck. </p>
  
  <p>How can I include a legend that labels each of the lines separately? I'd like to include the moderator variable's name as well as "-1 SD", "Mean", and "+1 SD" as the labels.</p>
  
  <p><code>dput</code> of the <code>ablines</code> data:</p>
  
  <pre><code>structure(list(moderator_value = c(-1, 0, 1), simple_intercept = c(-0.0274552251655293, 
  0.0599069333124192, 0.147269091790368), simple_slope = c(0.276897278474258, 
  0.214782863579552, 0.152668448684846)), .Names = c("moderator_value", 
  "simple_intercept", "simple_slope"), row.names = c(NA, 3L), class = "data.frame")
  </code></pre>