与cowplot和plot_grid共享面板边框

Question

我想在使用cowplot包中的plot_grid对齐的两个图表周围绘制边框。 请参见以下示例（改编自"更改轴位置"手册）：

require(gtable)
require(cowplot)

# top plot
p1 <- ggplot(mtcars, aes(mpg, disp)) + geom_line(colour = 'blue') +
  background_grid(minor = 'none')
g1 <- switch_axis_position(p1, 'xy') # switch both axes
g1 <- gtable_squash_rows(g1, length(g1$height)) # set bottom row to 0 height

# bottom plot
p2 <- ggplot(mtcars, aes(mpg, qsec)) + geom_line(colour = 'green') + ylim(14, 25) +
  background_grid(minor = 'none')
g2 <- ggplotGrob(p2)
g2 <- gtable_add_cols(g2, g1$widths[5:6], 4) # add the two additional columns that g1 has
g2 <- gtable_squash_rows(g2, 1:2) # set top two rows to 0 height

plot_grid(g1, g2, ncol = 1, align = 'v') + 
  annotate("rect", xmin = 0.1, xmax = 0.9, ymin = 0.1, ymax = 0.9, 
           color = "red", fill = NA)

现在，我不想使用任意选择的坐标来定位红色框，而是希望它与轴线对齐。我认为这些坐标可以从plot_grid输出中提取，但我不知道如何做到。

- tractorjack

Answer 1

根据我对grobs的理解，我认为在使用plot_grid组合图之前，先获取每个图的坐标并添加边界线段会更容易一些。 示例数据:

library(gtable)
library(cowplot)

# sample plots
# (note: the cowplot function switch_axis_position has been deprecated, as its
# creator notes ggplot2 now natively supports axes on either side of the plot.)
p1 <- ggplot(mtcars, aes(mpg, disp)) + 
  geom_line(colour = 'blue') +
  scale_x_continuous(position = "top") +
  scale_y_continuous(position = "right") +
  background_grid(minor = 'none'); p1

p2 <- ggplot(mtcars, aes(mpg, qsec)) + 
  geom_line(colour = 'green') + 
  ylim(14, 25) +
  background_grid(minor = 'none'); p2

# convert to grob objects
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)

函数用于为每个绘图grob添加适当的边框段，其中：

• grob是通过ggplotGrob创建的grob对象；
• sides是包含任意排列组合的字符字符串"t"/ "l"/ "b"/ "r"，表示所需边框位置的方向;
• col是所需的边框颜色（默认为红色）；
• ... 用于将其他参数传递给segmentsGrob()中的gpar()

.

最初的回答:

这个函数是用来给每个绘图元素加边框的。其中，你需要输入grob对象（由ggplotGrob生成），sides是一个字符串，里面包含了"t", "l", "b",和"r"这四种字母的任意组合，代表需要加边框的方向。你可以选择边框的颜色，如果不指定，默认为红色。另外还可以使用"..."传入其他参数到segmentsGrob()中的gpar()函数。

library(grid)

add.segments <- function(grob, sides = "tlbr", col = "red", ...){

  # get extent of gtable cells to be surrounded by border
  panel.coords <- g1[["layout"]][g1[["layout"]][["name"]] == "panel", ]
  t <- if(grepl("t", sides)) panel.coords[["t"]] else 1
  b <- if(grepl("b", sides)) panel.coords[["b"]] else length(grob[["heights"]])
  l <- if(grepl("l", sides)) panel.coords[["l"]] else 1
  r <- if(grepl("r", sides)) panel.coords[["r"]] else length(grob[["widths"]])

  # define border coordinates, & filter for the desired border sides
  coords <- data.frame(direction = c("t", "b", "l", "r"),
                       x0 = c(0, 0, 0, 1), y0 = c(1, 0, 0, 0),
                       x1 = c(1, 1, 0, 1), y1 = c(1, 0, 1, 1),
                       stringsAsFactors = FALSE)
  coords <- coords[sapply(coords$direction, grepl, sides), ]

  # add desired border sides as segments to the grob at specific gtable cells
  grob <- gtable_add_grob(x = grob,
                          grobs = segmentsGrob(
                            x0 = coords[["x0"]], y0 = coords[["y0"]],
                            x1 = coords[["x1"]], y1 = coords[["y1"]],
                            gp = gpar(col = col, ...)
                          ),
                          t = t, l = l, b = b, r = r,
                          clip = "off", name = "segments")

  return(grob)
}

使用方法:

plot_grid(add.segments(g1, "tlr"), 
          add.segments(g2, "lbr"), 
          ncol = 1, align = "v")

最初的回答：

另一个例子，将两个图水平对齐（好吧，将这些特定的图并排放置没有意义，但您明白了）：

plot_grid(add.segments(g2, "tlb", col = "gold2", lty = 2, lwd = 5), 
          add.segments(g1, "trb", col = "gold2", lty = 2, lwd = 5), 
          nrow = 1, align = "h")

- Z.Lin