arrange grobs in a gtable from a layout specification

gtable_layout.r

gtable_layout <- function(grobs, widths = NULL, heights = NULL,
                          m, ...){

  if(is.null(widths))
    widths <- unit(rep(1,ncol(m)), "null")
  if(is.null(heights))
    heights <- unit(rep(1,nrow(m)), "null")

  cells <- sort(unique(c(m)))

  ## left/right/top/bottom borders for given id
  range_cell <- function(ii){
    ind <- which(m == ii, arr.ind=TRUE)
    c(l=min(ind[,"col"]),
               r=max(ind[,"col"]),
               t=min(ind[,"row"]),
               b=max(ind[,"row"]))
  }

  layout <- data.frame(t(sapply(cells, range_cell)))

  gt <- gtable(widths = widths, heights = heights, ...)

  gtable_add_grobs <- gtable_add_grob # alias
  with(layout, gtable_add_grobs(gt, grobs, t=t, l=l, b=b, r=r))

}

gtable_layout_test.r

gl <- lapply(1:9, function(ii) grobTree(textGrob(ii), rectGrob()))
gt <- gtable_layout(gl, m=rbind(c(1,1,1,2,3),
                                c(1,1,1,4,5),
                                c(6,7,8,9,9)))
grid.draw(gt)