White space generated in grid.arrange - r

Question

I need to arrange several plots for a figure. I am creating individual plots using base and grid graphics. In order to arrange them in a single figure I have been using grid.echo(), grid.grab() to convert to grobs and then arrangeGrob() and grid.arrange() to build the final figure. A few weeks ago my tentative figure was working fine but now when I rerun the code it produces a figure with whitespace in the margins of the plots.
I add a minimal example that shows the problem that I am facing...
##minimal example
library(grid)
library(gridExtra)
library(gridGraphics)
##test plot
plot_n1<-plot(1:10,1:10, asp=1)
##convert test plot to grob
grid.echo()
test_p<-grid.grab()
##simulate several plots arranged in a more complex layout
multi<-arrangeGrob(test_p, test_p, test_p, test_p, ncol=1, heights=c(1/4,1/4,1/4,1/4))
##create graph
png(filename="minimal_multiplot.png", res=300, width=20, height=20, units="cm")
grid.arrange(test_p, multi, ncol=2, widths=c(2/3,1/3))
dev.off()
What am I doing wrong?

Answer 1

There does indeed appear to be a problem when converting a graphics plot into a grid plot, then using grid.grab() to grab and then draw the plot into a smaller regions (i.e., using your method). For instance, using viewports to define a slightly smaller region (coloured grey in the image below), axis material is missing.
# Packages
library(grid)
library(gridGraphics)
plot(1:10,1:10)
grid.echo()
test_p = grid.grab()
grid.newpage()
pushViewport(viewport(x = 0, width = .85, just = "left"))
grid.rect(gp = gpar(col = NA, fill = "grey90"))
grid.draw(test_p)
upViewport()
grid.rect(gp = gpar(col = "grey90", size = .1, fill = NA))
But Paul Murrell (author of the gridGraphics package) offers an alternative (see the examples at ?gridGraphics::grid.echo, and pp. 156-157 of The gridGraphics package in The R Journal v7/1). One can define a function that draws the plot, then that function becomes the argument of grid.echo() at the time of drawing the plot within the viewport. newpage = FALSE stops grid from opening a new page. Note that none of the axis material is chopped off.
pf = function() {
plot(1:10,1:10)
}
grid.newpage()
pushViewport(viewport(x = 0, width = .85, just = "left"))
grid.rect(gp = gpar(col =NA, fill = "grey90"))
grid.echo(pf, newpage=FALSE)
upViewport()
grid.rect(gp = gpar(col = "grey90", size = .1, fill = NA))
So to get your desired plot, I would do something like this - but still using viewports.
pf = function() {
par(mar=c(7.2, 7.2, 1, 1), mex = .3, tcl = .15, mgp = c(3, .15, 0))
plot(1:10, 1:10, cex.axis = .75, cex.lab = .75)
}
grid.newpage()
pushViewport(viewport(layout = grid.layout(3, 2,
widths = unit(c(2, 1), "null"),
heights = unit(c(1, 1, 1), "null"))))
pushViewport(viewport(layout.pos.col = 1, layout.pos.row = 1:3))
grid.echo(pf, newpage = FALSE)
upViewport()
for(i in 1:3) {
pushViewport(viewport(layout.pos.col = 2, layout.pos.row = i))
grid.echo(pf, newpage = FALSE)
upViewport()
}
upViewport()
grid.rect(gp = gpar(col = "grey90", size = .1, fill = NA))