title: How can I remove bar boarders in geom_histogram using the pdf device
body: <p>I only have the problem when I try to make a pdf! Not on the default device or with png!</p>
  
  <pre><code>library(ggplot2)
  theme_set(theme_bw())
  data(iris)
  pdf("test.pdf")
  ggplot(iris,aes(x=Sepal.Length)) + geom_histogram()
  dev.off()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/FUlRM.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/FUlRM.png" alt="results from test.pdf"></a></p>
  
  <p>What I want is what I get if I set the border color equal to the fill color. </p>
  
  <pre><code>pdf("test2.pdf")
  ggplot(iris,aes(x=Sepal.Length)) + geom_histogram(color='grey35')
  dev.off()
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/JUwgG.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/JUwgG.png" alt="Results from temp2.pdf  "></a></p>
  
  <p>Of course that solution won't generalize if I change color themes, or use the fill aesthetic. </p>