Add new columns in specified position.

rect <- rectGrob(gp = gpar(fill = "#00000080"))
tab <- gtable(unit(rep(1, 3), "null"), unit(rep(1, 3), "null"))
tab <- gtable_add_grob(tab, rect, t = 1, l = 1, r = 3)
tab <- gtable_add_grob(tab, rect, t = 1, b = 3, l = 1)
tab <- gtable_add_grob(tab, rect, t = 1, b = 3, l = 3)
dim(tab)

## [1] 3 3

plot(tab)

# Grobs will continue to span over new rows if added in the middle
tab2 <- gtable_add_cols(tab, unit(1, "null"), 1)
dim(tab2)

## [1] 3 4

plot(tab2)

# But not when added to left (0) or right (-1, the default)
tab3 <- gtable_add_cols(tab, unit(1, "null"))
tab3 <- gtable_add_cols(tab3, unit(1, "null"), 0)
dim(tab3)

## [1] 3 5

plot(tab3)