将“facet_grid”从多面画布上拉开？

考虑以下示例?ggplot2::facet_grid:

p <- ggplot(mpg, aes(displ, cty)) + geom_point()
p + facet_grid(vars(drv), vars(cyl))

Which results in the following image

What I would like to achieve is a minor offset/white space between the facet-labeling strips and the actual plots. The result would look (sans the gimp involvement):

是否可以使用ggplot2或其任何衍生包？

感谢您的任何见解。

遵循上面@Tung的回答（以及一些忽略文档的实验），我让它工作：

p <- ggplot(mpg, aes(displ, cty)) + geom_point() 
p +
  facet_grid(vars(drv), vars(cyl)) +
  theme(strip.switch.pad.grid = unit(0.2, "cm"), strip.placement = "outside")

还有...... tada！：