gtable

a <- gtable(unit(1:3, c("cm")), unit(5, "cm"))
a

## TableGrob (1 x 3) "layout": 0 grobs

gtable_show_layout(a)

# Add a grob:
rect <- rectGrob(gp = gpar(fill = "black"))
a <- gtable_add_grob(a, rect, 1, 1)
a

## TableGrob (1 x 3) "layout": 1 grobs
##   z     cells   name               grob
## 1 1 (1-1,1-1) layout rect[GRID.rect.17]

plot(a)

# gtables behave like matrices:
dim(a)

## [1] 1 3

t(a)

## TableGrob (3 x 1) "layout": 1 grobs
##   z     cells   name               grob
## 1 1 (1-1,1-1) layout rect[GRID.rect.17]

plot(t(a))

# when subsetting, grobs are retained if their extents lie in the
# rows/columns that retained.

b <- gtable(unit(c(2, 2, 2), "cm"), unit(c(2, 2, 2), "cm"))
b <- gtable_add_grob(b, rect, 2, 2)
b[1, ]

## TableGrob (1 x 3) "layout": 0 grobs

b[, 1]

## TableGrob (3 x 1) "layout": 0 grobs

b[2, 2]

## TableGrob (1 x 1) "layout": 1 grobs
##   z     cells   name               grob
## 1 1 (1-1,1-1) layout rect[GRID.rect.17]

# gtable have row and column names
rownames(b) <- 1:3
rownames(b)[2] <- 200
colnames(b) <- letters[1:3]
dimnames(b)

## [[1]]
## [1]   1 200   3
## 
## [[2]]
## [1] "a" "b" "c"