Experiments with grid.draw

library(grid)
one <- unit(1, "cm")

# rectGrob with no fill
grob1 <- rectGrob(width = one, height = one)

# gTree of recGrob, with blue fill
test <- gTree(children = gList(grob1), cl = "test", gp = gpar(fill = "blue"))

# draw rectGrob: No fill
grid.newpage()
grid.draw(grob1)

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# draw gTree: Blue fill
grid.newpage()
grid.draw(test)

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# Paul Murrell's code: This essentially calls grid.draw drawing each child
# in the gTree Has the same effect as the first plot (and ignores the gp
# in the gTree, I think)
grid.draw.test <- function(x, ...) {
    lapply(x$children, grid.draw)
}
grid.newpage()
# NO blue fill
grid.draw(test)

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## $GRID.rect.1
## NULL
##

widthDetails() and grobWidth()

Here's what happens when you call widthDetails() and grobWidth() on the rectGrob and gTree objects:.

# widthDetails of rectGrob
widthDetails(grob1)

## [1] 0.393700787401575inches

convertUnit(widthDetails(grob1), "cm")

## [1] 0.999999999999999cm


# grobWidth of gTree
grobWidth(grob1)

## [1] 1grobwidth

convertUnit(grobWidth(grob1), "cm")

## [1] 0.999999999999999cm



# widthDetails of gTree
widthDetails(test)

## [1] 1null

convertUnit(widthDetails(test), "cm")

## [1] 0cm


# grobWidth of gTree
grobWidth(test)

## [1] 1grobwidth

convertUnit(grobWidth(test), "cm")

## [1] 0cm