title: Axis labels run into axis when exporting plots with negative tick mark length (ggplot2)
body: <p>I'm having some trouble exporting a simple scatterplot to a pdf file in ggplot2. Specifically when I set the tick mark length to a negative value (essential), the tick mark labels merge with the axis line (as shown below).</p>
  
  <p><img src="https://i.stack.imgur.com/RcpxP.png" alt="Problem tick mark labels"></p>
  
  <p>The figure appears normally in the plot window of rstudio but when I export to pdf the problem occurs. Altering <code>vjust</code> in <code>axis.text.x</code> doesn't seem to be helping at all. I've also tried manipulating the plot margins. </p>
  
  <p>Is anyone aware of a way of moving the labels away from the axis in such a situation?</p>
  
  <p>Hopefully the code below should replicate the issue.</p>
  
  <pre><code>data = data.frame(xvar = seq(1:20), yvar = seq(1:20), labvar = rep(c("A", "B"), 10))
  
  require(ggplot2)
  require(gridExtra)
  p &lt;- ggplot(data = data, aes(x = xvar, y = yvar)) +  geom_point() +
    facet_wrap(~labvar, scales = "fixed") +    
    theme_classic()+
    theme(axis.ticks.length=unit(-0.1, "cm"),
    axis.text.x=element_text(vjust = 0))
  
  p
  </code></pre>