Arranging multiple grobs on a page

Baptiste Auguie

2019-12-31

The grid package provides low-level functions to create graphical objects (grobs), and position them on a page in specific viewports. The gtable package introduced a higher-level layout scheme, arguably more amenable to user-level interaction. With the arrangeGrob/grid.arrange() pair of functions, gridExtra builds upon gtable to arrange multiple grobs on a page.

Basic usage

In this example we mix a few grobs and plots,

library(gridExtra)
library(grid)
library(ggplot2)
library(lattice)
p <- qplot(1,1)
p2 <- xyplot(1~1)
r <- rectGrob(gp=gpar(fill="grey90"))
t <- textGrob("text")
grid.arrange(t, p, p2, r, ncol=2)

Title and/or annotations

Complex layouts

We can provide a matrix defining the layout,

lay <- rbind(c(1,1,1,2,3),
             c(1,1,1,4,5),
             c(6,7,8,9,9))
grid.arrange(grobs = gs, layout_matrix = lay)

The layout itself may contain holes, but note that for any given grob index the region must be simply connected (no hole),

hlay <- rbind(c(1,1,NA,2,3),
              c(1,1,NA,4,NA),
              c(NA,7,8,9,NA))
select_grobs <- function(lay) {
  id <- unique(c(t(lay))) 
  id[!is.na(id)]
} 
grid.arrange(grobs=gs[select_grobs(hlay)], layout_matrix=hlay)

All cells are of equal size by default, but users may pass explicity widths and/or heights in any valid grid units, or as relative numbers (interpreted as null),

grid.arrange(grobs=gs[1:3], ncol=2, widths = 1:2, 
             heights=unit(c(1,10), c("in", "mm")))

Nested layouts with arrangeGrob

The grid.arrange() function draws on the device; for more complex layouts, we may want to store the gtable and combine it with other objects, e.g. forming nested layouts. To this end, use arrangeGrob(),

g1 <- arrangeGrob(grobs = gs, layout_matrix = t(lay))
g2 <- arrangeGrob(grobs = gs, layout_matrix = lay)
grid.arrange(g1, g2, ncol=2)

Multiple pages output

Finally, we may want to place grobs on multiple pages; the marrangeGrob() function provides a convenient interface for this, also compatible with ggsave().