r - ggplot2 0.9.2中的数学符号

要将数学符号添加到facet_wrap时，您曾经能够像(example taken from here)中一样使用grid封装：

library(ggplot2); library(grid)
d <- ggplot(diamonds, aes(carat, price, fill = ..density..))
    xlim(0, 2) + stat_binhex(na.rm = TRUE) + opts(aspect.ratio = 1)
d <- d + facet_wrap(~ color, ncol = 4)
grob <- ggplotGrob(d)
strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name
grob <- editGrob(grob, strip_elem[1], label=expression(Y[1]))
grid.draw(grob)

这不再起作用，因为它给出了此错误：

>  strip_elem <- grid.ls(getGrob(grob, "strip.text.x", grep=TRUE, global=TRUE))$name
Error in getGrob(grob, "strip.text.x", grep = TRUE, global = TRUE) :
  It is only valid to get a child from a 'gTree'
> grob <- editGrob(grob, strip_elem[1], label=expression(Y[1]))
Error in editGrob(grob, strip_elem[1], label = expression(Y[1])) :
  object 'strip_elem' not found

我们如何在0.9.2版中添加数学符号？

最佳答案

更新使用facet_wrap_labeller函数available from here可能会更容易。

library(ggplot2)
library(gtable)

d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
    xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1)
(d <- d + facet_wrap(~color, ncol = 4))

facet_wrap_labeller <- function(gg.plot, labels=NULL) {
  require(gridExtra)

  g <- ggplotGrob(gg.plot)
  gg <- g$grobs
  strips <- grep("strip_t", names(gg))

  for(ii in seq_along(labels))  {
    modgrob <- getGrob(gg[[strips[ii]]], "strip.text",
                       grep=TRUE, global=TRUE)
    gg[[strips[ii]]]$children[[modgrob$name]] <- editGrob(modgrob,label=labels[ii])
  }

  g$grobs <- gg
  class(g) = c("arrange", "ggplot",class(g))
  return(g)
}

# Some arbitrary strip texts
StripTexts = expression(Y[1], E, F, G, H, I, J)

# Apply the facet_wrap_labeller function
g = facet_wrap_labeller(d, StripTexts)

# Draw it
g

原始解决方案，仅需进行少量编辑即可使带状文本的选择更加轻松。

这行得通，但中间有点混乱。它使用ggplot2 0.9.2以及grid和gtable包中的函数。必须有一种更简单的方法，我希望您可以从中获得一些东西来制定一个更简单的解决方案。

这些步骤是：


绘制原始图；
找到需要更改的带状元素；
提取带状元素；
将“ D”更改为expression(Y[1])；
将修改后的带状元素重新插入到原始图中。

library(ggplot2); library(grid); library(gtable)
d <- ggplot(diamonds, aes(carat, price, fill = ..density..)) +
    xlim(0, 2) + stat_binhex(na.rm = TRUE) + theme(aspect.ratio = 1)
(d <- d + facet_wrap(~color, ncol = 4))
# The aim is to change the strip text "D" to expression(Y[1])

# Winston's accessor function:
gt_getgrob <- function(gt, pattern) {
  idx <- grep(pattern, gt$layout$name)
  if (length(idx) > 1)
    stop("More than one match for pattern '", pattern, "'")
  gt$grobs[[idx]]
}

g = ggplotGrob(d)
g$layout    # We want "strip_t-1"
strip <- gt_getgrob(g, "t-1")        # Use the accessor function to extract it.
str(strip)     # Locate the "D" label

strip$children[[2]]$label = expression(Y[1])    # Change the label.
grid.draw(strip)       # Yes, it's worked

gt <- ggplot_gtable(ggplot_build(d))
gtable_show_layout(gt)       # Get the layout of the original plot.

gt = gtable_add_grob(gt, strip, t=3, l=4, b=3, r=4)  # Insert the modified strip element into the plot.
grid.newpage()
grid.draw(gt)

结果是：