title: ggplot2: Change legend symbol
body: <p>I have plotted several lines and I am wondering how to change the symbol in the legend to go from the thin line to a full block.</p>
  
  <p>I am trying to go from </p>
  
  <p><img src="https://i.imgur.com/Pg5i1nL.png" alt="this"> to <img src="https://i.imgur.com/iLXWrD1.png" alt="this"> </p>
  
  <p>(while using geom_line and not geom_bar)</p>