title: ggpairs fill histograms with white
body: <p>I would like to get the histograms or bar line in the following plot in white. It's mainly for ink saving concerns.</p>
  
  <pre><code>library(GGally)
  library(ggplot2)
  data(tips, package = "reshape")
  pm &lt;- ggpairs(tips[,1:3])
  </code></pre>
  
  <p>I've tried </p>
  
  <pre><code>pm + theme(panel.background = element_rect(fill = "white"),
  plot.background = element_rect(fill = "white"),rect= element_rect(fill="white"))
  </code></pre>
  
  <p>but couldn't reach the histogram fill.</p>
  
  <p>Any help?</p>