title: Control ggplot2 legend look without affecting the plot
body: <p>I'm plotting lines with ggplot2 like this:</p>
  
  <pre><code>ggplot(iris, aes(Petal.Width,Petal.Length,color=Species)) + geom_line() + theme_bw()
  </code></pre>
  
  <p><img src="https://i.imgur.com/iQK92Ja.png" alt="current plot">.</p>
  
  <p>I find legend marks to be small so I want them to be bigger. If I change the size, lines on the plot change too:</p>
  
  <pre><code>ggplot(iris, aes(Petal.Width,Petal.Length,color=Species)) + geom_line(size=4) + theme_bw()
  </code></pre>
  
  <p><img src="https://i.imgur.com/J38oSKR.png" alt="thick plot lines">.</p>
  
  <p>But I only want to see thick lines in the legend, I want lines on the plot to be thin. I tried to use <code>legend.key.size</code> but it changes the square of the mark, not the width of the line:</p>
  
  <pre><code>library(grid)  # for unit
  ggplot(iris,aes(Petal.Width,Petal.Length,color=Species))+geom_line()+theme_bw() + theme(legend.key.size=unit(1,"cm"))
  </code></pre>
  
  <p><img src="https://i.imgur.com/rYR2dfk.png" alt="big legend keys"></p>
  
  <p>I also tried to use points:</p>
  
  <pre><code>ggplot(iris,aes(Petal.Width,Petal.Length,color=Species)) + geom_line() + geom_point(size=4) + theme_bw()
  </code></pre>
  
  <p>But of course it still affects both plot and legend:</p>
  
  <p><img src="https://i.imgur.com/8m61EUE.png" alt="points"></p>
  
  <p>I wanted to use lines for the plot and dots/points for the legend.</p>
  
  <p>So I'm asking about two things:</p>
  
  <ol>
  <li>How to change width of line in the legend without changing the plot?</li>
  <li>How to draw lines in the plot, but draw points/dots/squares in the legend?</li>
  </ol>