title: making a single gridline thicker in ggplot2
body: <p>I have a plot of a bunch of values between -1 and 1.  Let's say it looks like this:</p>
  
  <pre><code>data &lt;- data.frame(x=1:20,y=runif(20)*2-1)
  p &lt;- ggplot(data=data,aes(x=x,y=y)) + geom_line() + scale_y_continuous(breaks=seq(-1,1,0.5),limits=c(-1,1))
  p
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/Hl6XG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Hl6XG.png" alt="enter image description here"></a></p>
  
  <p>What I want is for the horizontal line with y=0, to be thicker than all the other gridlines.  Is there a simple way to do this?</p>