title: Grayscale line graph and one line in red
body: <p>I would like to create a grayscale line-graph with multiple groups (~10), but having one line <code>(spec=3)</code> that is in red. </p>
  
  <p>See an example with only 3 groups below:</p>
  
  <pre><code>year &lt;-c (2011, 2012, 2013, 2011, 2012, 2013, 2011, 2012, 2013)
  x &lt;- 1:10
  cost &lt;- sample(x, 9, replace=T)
  spec &lt;- as.factor(c(1, 1, 1, 2, 2, 2, 3, 3, 3))
  
  dat &lt;-data.frame(year=year, cost=cost, spec=spec)
  
  # graph
  library(ggplot2)
  ggplot(data=dat, aes(x=year, y=cost, group=spec)) +
    geom_line(aes(color=spec)) + 
    **geom_line(group="3", col="red")** + 
    scale_colour_grey() + 
    theme_bw()
  </code></pre>
  
  <p>The problem is obviously with the <code>geom_line(group="3", col="red")</code> part, but I don't know how to fix it. </p>
  
  <p>With this code I get something strange like this:
  <a href="https://i.stack.imgur.com/GgKZV.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GgKZV.png" alt="enter image description here"></a></p>