Axis labels run into axis when exporting plots with negative tick mark length (ggplot2)

Question

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).

The figure appears normally in the plot window of rstudio but when I export to pdf the problem occurs. Altering vjust in axis.text.x doesn't seem to be helping at all. I've also tried manipulating the plot margins.

Is anyone aware of a way of moving the labels away from the axis in such a situation?

Hopefully the code below should replicate the issue.

data = data.frame(xvar = seq(1:20), yvar = seq(1:20), labvar = rep(c("A", "B"), 10))

require(ggplot2)
require(gridExtra)
p <- 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

Answer 1

Try using axis.ticks.margin:

p + theme(axis.ticks.margin = unit(5, "lines"))

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I notice that the documentation for ?theme now includes a list of all theme elements and what they do.