How to plot grid plots on a same page?

I am using a package (treemap) that uses grid package to produce a treemap. However, I would like to plot several of these treemaps together, to add different color schemes to these plots. tmPlot function uses grid.newpage function, which clears the graphics window. I have not found a way to save grid.newpage objects as you can do for ggplot2objects. Is there a way to plot several grid.newpage objects to a same window?

## Example

library(treemap)

# load Gross national income data
data(GNI2010)

size <- aggregate(GNI ~ continent, GNI2010, sum)
size <- size[with(size, order(GNI, decreasing = T)),]

cont <- size$continent

widths <- c(sum(size[c(1,3,5),]$GNI),
sum(size$GNI) - sum(size[c(1,3,5),]$GNI))

heights <- c(sum(size[c(1,2),]$GNI),
sum(size[c(3,4),]$GNI),
sum(size[c(5,6),]$GNI))

palettes <- c("Greens", "Blues", "Reds", "Oranges", "Purples", "Greys")

i <- 1 # This is to be replaced by for loop

x <- subset(GNI2010, continent == cont[i], cex = 5)

# create treemap

layout(matrix(1:6, 3, byrow = TRUE), widths = widths, heights = heights)
x1 <- tmPlot(x,
index=c("iso3"),
vSize="population",
vColor="GNI",
type="value", title = "", 
position.legend = "none",
palette = palettes[i])
grid.text(cont[i], 0.5, 0.5, gp=gpar(fontsize=20, font = 2, col = "white"))

## x1 is does not make a plot as such and tmPlot overwrites layout

I understand that my solution to scale the plots based on GNI sum is not right. I might make another question about that later, once I figure out how to plot these treemaps in a same window.

EDIT: I think the answer to this question is "no". Currently you cannot save grid.newpage objects by name, neither can you save several of these on a page, because the function "erases the current device or moves to a new page" as said in the description. However, it is possible to find work arounds. tmPlot package does not currently (as of 23 March, 2013) support viewports, but the development version does.

Solution

Thanks for your question. The output of tmPlot is indeed not a saved plot.

In the next update I will add argument vp, by which a viewport can be specified to draw in. Only if it is not specified, grid.newpage is called.

UPDATE: You could check and test the development version at https://github.com/mtennekes/treemap

To apply the example of Bryan Hanson:

vplayout <- function(x, y) viewport(layout.pos.row = x, layout.pos.col = y)
grid.newpage()
pushViewport(viewport(layout = grid.layout(1, 2)))
tmPlot(GNI2010,
  index="continent",
  vSize="population",
  vColor="GNI",
  type="value",
  vp = vplayout(1,1))
tmPlot(GNI2010,
  index=c("continent", "iso3"),
  vSize="population",
  vColor="GNI",
  type="value",
  vp = vplayout(1,2))