title: Add regression line legend to geom_abline
body: <p>I've coded this:</p>
  
  <pre><code>ggplot() + 
    geom_point(mapping = aes(x = X, y = y)) + 
      geom_abline(intercept = -0.9930872, slope = 0.4866284, colour = "red") + 
      geom_abline(intercept = -1, slope = 0.5, colour = "blue")
  </code></pre>
  
  <p>but cannot seem to get a working legend for my least square and populuation regression line. I've tried various stack overflow answers but nothing seems to give me what I need. </p>
  
  <p><a href="https://stackoverflow.com/questions/36695984/add-a-legend-to-a-ggplot2-scatter-plot-including-additional-lines">Add a legend to a ggplot2 scatter plot including additional lines</a></p>
  
  <p>This looked like the best answer, but I can't get it to work!</p>
  
  <p>Any suggestions?</p>