gtable_add_grob（）：linesGrob（）not displayed

Question

爲什麼這些行（通過linesGrob）未繪製在下面的圖中？

require(gtable) 
base <- gtable(widths=unit(rep(1, 2), "null"), 
       heights=unit(rep(1, 3), "null")) 
grid.newpage() 
g <- 1 
for(i in 1:3) { 
    for(j in 1:2) { 
     base <- gtable_add_grob(base, 
           grobs=list(linesGrob(x=1:4, y=4:1), 
              rectGrob(gp=gpar(fill="#FF0000")), 
              textGrob(label=g)), i, j, name=1:3) 
     g <- g+1 
    } 
} 
grid.draw(base) 

Answer 1

兩個原因：

• 座標檢視區以外

• 的rectGrob繪製在頂部和口罩它

---

require(gtable) 
# let's fix this name before it's too late 
gtable_add_grobs <- gtable_add_grob 

base <- gtable(widths=unit(rep(1, 2), "null"), 
       heights=unit(rep(1, 3), "null")) 
grid.newpage() 
g <- 1 
for(i in 1:3) { 
    for(j in 1:2) { 
     base <- gtable_add_grobs(base, 
           grobs=list(rectGrob(gp=gpar(fill="#FF0000")), 
              linesGrob(x=1:4, y=4:1, def="native", 
                vp=dataViewport(1:4, 1:4)), 
              textGrob(label=g)), i, j, name=1:3) 
     g <- g+1 
    } 
} 
grid.draw(base) 

---

注意gtable_add_grobs被矢量化，這意味着你不應該使用for循環，理想情況下。如果你把所有的grobs先組合在一起，對於一個給定的單元格，在gTree中更容易。這裏有一個簡化版本，

library(gtable) 

g <- gtable(widths = unit(c(1,1), "null"), 
      heights = unit(c(1,1), "null")) 


cell <- function(ii) 
    grobTree(rectGrob(), 
      linesGrob(1:4, 1:4, default.units="native"), 
      textGrob(ii), 
      vp=dataViewport(c(1,4), c(1,4))) 

gl <- lapply(1:4, cell) 

xy <- expand.grid(1:2, 1:2) 

g <- gtable_add_grobs(g, gl, 
         l=xy[,1], 
         r=xy[,1], 
         t=xy[,2], 
         b=xy[,2]) 


grid.newpage() 
grid.draw(g)