title: Increase the size of line in geom_line
body: <p>This is the code I am using:</p>
  
  <pre><code>ggplot(data, aes(x = Date1, group=1)) + 
    geom_line(aes(y = Wet, colour = "Wet")) + 
    geom_line(aes(y = Dry, colour = "Dry"))
  </code></pre>
  
  <p>When I use the function size, the lines are too thick and their width is identical from <code>size=0.1</code> to <code>size=10</code> or more. Is there a way to control the size of the line?</p>
  
  <p>Dummy data:</p>
  
  <pre><code>Date         Wet    Dry
  July        5.65    4.88
  September   5.38    3.93
  October     4.73    2.42
  </code></pre>