Как «пересечь» ячейку в таблице без имен

Question

Когда я пытаюсь адаптировать пример из виньетки в gridExtra, а именно

g <- tableGrob(iris[1:4, 1:3])
g <- gtable_add_grob(g,
        grobs = grobTree(
            segmentsGrob( # diagonal line ul -> lr
                x0 = unit(0,"npc"),
                y0 = unit(1,"npc"),
                x1 = unit(1,"npc"),
                y1 = unit(0,"npc"),
                gp = gpar(lwd = 2.0)),
            segmentsGrob( # diagonal line ll -> ur
                x0 = unit(0,"npc"),
                y0 = unit(0,"npc"),
                x1 = unit(1,"npc"),
                y1 = unit(1,"npc"),
                gp = gpar(lwd = 2.0))),
        t = 3, b = 3, l = 3, r = 3
    )
grid.draw(g)

вВ моем наборе данных, в котором нет имен строк или столбцов, крест не отображается:

g <- tableGrob(iris[1:4, 1:3], rows=NULL, cols=NULL) # the only line that is changed
g <- gtable_add_grob(g,
        grobs = grobTree(
            segmentsGrob( # diagonal line ul -> lr
                x0 = unit(0,"npc"),
                y0 = unit(1,"npc"),
                x1 = unit(1,"npc"),
                y1 = unit(0,"npc"),
                gp = gpar(lwd = 2.0)),
            segmentsGrob( # diagonal line ll -> ur
                x0 = unit(0,"npc"),
                y0 = unit(0,"npc"),
                x1 = unit(1,"npc"),
                y1 = unit(1,"npc"),
                gp = gpar(lwd = 2.0))),
        t = 3, b = 3, l = 3, r = 3
    )
grid.draw(g)

Я застрял.Любая помощь?