r - 以给定的纵横比保存绘图

Question

我正在使用非常棒的库 ggplot2。我想出了如何使用 coord_fixed 设置绘图的纵横比。现在，我想将绘图保存为具有指定宽度(例如 10 厘米)的 PDF，并计算所需的高度。我不知道如何实现这一目标。这可能吗？

Answer 1

您可以使用网格函数来计算 ggplot grob 的完整大小，但有(编辑: 至少)两个注意事项:

• 将打开一个额外的设备窗口，进行单位转换

• 绘图面板大小默认为 0，因为它是根据它所在的设备(视口(viewport))即时计算的，而不是相反。

也就是说，以下函数尝试打开一个完全适合 ggplot 的设备，

library(ggplot2)
library(grid)

sizeit <- function(p, panel.size = 2, default.ar=1){

  gb <- ggplot_build(p)
  # first check if theme sets an aspect ratio
  ar <- gb$plot$coordinates$ratio

  # second possibility: aspect ratio is set by the coordinates, which results in 
  # the use of 'null' units for the gtable layout. let's find out
  g <- ggplot_gtable(gb)
  nullw <- sapply(g$widths, attr, "unit")
  nullh <- sapply(g$heights, attr, "unit")

  # ugly hack to extract the aspect ratio from these weird units
  if(any(nullw == "null"))
    ar <- unlist(g$widths[nullw == "null"]) / unlist(g$heights[nullh == "null"])

  if(is.null(ar)) # if the aspect ratio wasn't specified by the plot
       ar <- default.ar

  # ensure that panel.size is always the larger dimension
  if(ar <= 1 ) panel.size <- panel.size / ar

  g$fullwidth <- convertWidth(sum(g$widths), "in", valueOnly=TRUE) + 
    panel.size
  g$fullheight <- convertHeight(sum(g$heights), "in", valueOnly=TRUE) + 
    panel.size / ar

  class(g) <- c("sizedgrob", class(g))
  g
}


print.sizedgrob <- function(x){
  # note: dev.new doesn't seem to respect those parameters
  # when called from Rstudio; in this case it 
  # may be replaced by x11 or quartz or ...
  dev.new(width=x$fullwidth, height=x$fullheight)
  grid.draw(x)
}


p1 <- ggplot(mtcars, aes(x = wt, y = mpg)) + geom_point() + coord_fixed() +
  theme(plot.background = element_rect(colour = "red"))

p2 <- p1 + aes(x = mpg, y = wt)

# need for an explicit dummy device open, otherwise it's a bit off
# for no apparent reason that I can understand
dev.new() 

sizeit(p1, 0.1)

sizeit(p2, 2)