Getting the height of the xaxisGrob - r

Question

The grid-package has a special tool for generating axis. It writes the axis outside the current viewport and the height using grobHeight is therefore considered to be 0mm. Unfortunately I need to make space for the axis and I want to know the exact height of the object. Below is an example that illustrates the problem:
library(grid)
plotColorBar <- function () {
grid.newpage()
xg <- xaxisGrob(at=c(0,.25,.5,.75, 1),
label= sprintf("%d %%", c(0,.25,.5,.75, 1)*100),
main=FALSE)
bar_layout <- grid.layout(nrow=3, ncol=3,
heights = unit.c(unit(.80, "npc"),
grobHeight(xg),
unit(.2, "npc") - grobHeight(xg)),
widths = unit.c(unit(.25, "npc"),
unit(1, "npc") -
unit(.5, "npc"),
unit(.25, "npc")))
pushViewport(viewport(layout=bar_layout, name="Bar_layout"))
pushViewport(viewport(layout.pos.row=3,
layout.pos.col=2,
name="Color_bar"))
grid.draw(xg)
bar_clrs <- colorRampPalette(c("red", "blue"), space="Lab")(101)
grid.raster(t(as.raster(bar_clrs)), width=1, height=1, interpolate=FALSE)
popViewport()
pushViewport(viewport(layout.pos.row=1,
layout.pos.col=1:3,
name="Main_exc_bar"))
grid.rect(gp=gpar(col="black", fill="#00000022"))
grid.text("Coool")
popViewport()
}
png(filename="axisWihtoutHeight.png", width=250, height=250, res=96)
plotColorBar()
dev.off()
Gives the following image:
Notice that the grey area covers the axis text. When I try convertY(grobHeight(xg), "mm") it returns 0mm.
Now applying the #baptiste recommended fix improves the image slightly:
heightDetails.xaxis = function(x) do.call(sum, lapply(x$children, grobHeight))
png(filename="axisWihtHeight.png", width=250, height=250, res=96)
plotColorBar()
dev.off()
As you can see the text for some reason is twice the height. Adjusting this manually through the following option although this does feel a little clumsy:
heightDetails.xaxis = function(x) {
grobHeight(x$children$ticks) +
grobHeight(x$children$labels) +
grobHeight(x$children$labels)
}
png(filename="axisWihtDoubleHeight.png", width=250, height=250, res=96)
plotColorBar()
dev.off()
Final solution
As suggested it seems that unit(1.5*cex, "line") does the job nicely:
heightDetails.xaxis = function(x) {
cex <- 1
if (!is.null(x$children$labels$gp$cex))
cex <- x$children$labels$gp$cex
grobHeight(x$children$ticks) +
unit(1.5*cex, "line")
}

Answer 1

you'll want to define a heightDetails method for the gTree, something along those lines
heightDetails.xaxis = function(x) do.call(sum, lapply(x$children, grobHeight))