How can I reduce the size of the title margin or padding (the grey regions circled in the graph below). I can shrink the title text size, but the title box size/padding is too large in plots with many facets.
ggplot(mtcars, aes(x=disp, y=mpg)) +
facet_grid(. ~ cyl, scale="free_x") +
geom_point() +
theme(strip.text.x=element_text(size=5))