title: ggplot2: force legend to have one row when guides does not appear to have an effect
body: <p>I have the following toy example:</p>
  
  <pre><code>library(ggplot2)
  g &lt;- ggplot(mpg, aes(class))
  mpg$drv &lt;- sample(as.character((-4:4)), size = length(mpg$drv), replace = T)
  g + geom_bar(aes(fill = drv), position = "fill") + theme(legend.position = "bottom") + guides(colour = guide_legend(nrow = 1))
  </code></pre>
  
  <p>which for me<a href="https://i.stack.imgur.com/fwrZG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/fwrZG.png" alt="enter image description here"></a> yields the following figure. </p>
  
  <p>Try as I might, I am unable to get the legend at the bottom to be in one row. I looked around and the <code>guide_legend</code> appears to be having no effect. Is there a better way to do this, or is there another way to force the legend to be in one row?</p>