title: How to change angle of line in customized legend in ggplot2
body: <p>I'm trying to add a customized legend to my ggplot but the legend boxes have lines at an angle. I want to change that angle to 0 degrees. Is there any way to do that? Following is the code for an example plot.</p>
  
  <pre><code>ggplot()+geom_abline(aes(color="black",,slope=1,intercept = 0))+
     geom_abline(aes(color="red",slope=0.5,intercept = 0))+
     scale_color_manual(values=c("black"="black","red"="red"))
  </code></pre>
  
  <p>We can see that the lines in the legend boxes are slightly inclined and I want to make them horizontal.
  <a href="https://i.stack.imgur.com/Q7uKG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/Q7uKG.png" alt="enter image description here"></a></p>