如何管理 gtable() 的 t、b、l、r 坐标以正确绘制次要 y 轴的标签和刻度线
How to manage the t, b, l, r coordinates of gtable() to plot the secondary y-axis's labels and tick marks properly
我正在使用 facet_wrap 并且还能够绘制次要 y 轴。然而，标签并没有绘制在轴附近，而是绘制得很远。我意识到如果我了解如何操纵 grobs 的 gtable (t,b,l,r) 的坐标系，这一切都会得到解决。有人可以解释他们实际描述的方式和内容 - t:r = c(4,8,4,4) 是什么意思。
辅助 yaxis 与 ggplot 有很多链接，但是当 nrow/ncol 大于 1 时，它们会失败。所以请教我网格几何和 grobs 位置管理的基础知识。
编辑：代码
this is the final code written by me :
library(ggplot2)
library(gtable)
library(grid)
library(data.table)
library(scales)
# Data
diamonds$cut <- sample(letters[1:13], nrow(diamonds), replace = TRUE)
dt.diamonds <- as.data.table(diamonds)
d1 <- dt.diamonds[,list(revenue = sum(price),
stones = length(price)),
by=c("clarity", "cut")]
setkey(d1, clarity, cut)
# The facet_wrap plots
p1 <- ggplot(d1, aes(x = clarity, y = revenue, fill = cut)) +
geom_bar(stat = "identity") +
labs(x = "clarity", y = "revenue") +
facet_wrap( ~ cut) +
scale_y_continuous(labels = dollar, expand = c(0, 0)) +
theme(axis.text.x = element_text(angle = 90, hjust = 1),
axis.text.y = element_text(colour = "#4B92DB"),
legend.position = "bottom")
p2 <- ggplot(d1, aes(x = clarity, y = stones, colour = "red")) +
geom_point(size = 4) +
labs(x = "", y = "number of stones") + expand_limits(y = 0) +
scale_y_continuous(labels = comma, expand = c(0, 0)) +
scale_colour_manual(name = '', values = c("red", "green"),
labels = c("Number of Stones"))+
facet_wrap( ~ cut) +
theme(axis.text.y = element_text(colour = "red")) +
theme(panel.background = element_rect(fill = NA),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
panel.border = element_rect(fill = NA, colour = "grey50"),
legend.position = "bottom")
# Get the ggplot grobs
xx <- ggplot_build(p1)
g1 <- ggplot_gtable(xx)
yy <- ggplot_build(p2)
g2 <- ggplot_gtable(yy)
nrow = length(unique(xx$panel$layout$ROW))
ncol = length(unique(xx$panel$layout$COL))
npanel = length(xx$panel$layout$PANEL)
pp <- c(subset(g1$layout, grepl("panel", g1$layout$name), se = t:r))
g <- gtable_add_grob(g1, g2$grobs[grepl("panel", g1$layout$name)],
pp$t, pp$l, pp$b, pp$l)
hinvert_title_grob <- function(grob){
widths <- grob$widths
grob$widths[1] <- widths[3]
grob$widths[3] <- widths[1]
grob$vp[[1]]$layout$widths[1] <- widths[3]
grob$vp[[1]]$layout$widths[3] <- widths[1]
grob$children[[1]]$hjust <- 1 - grob$children[[1]]$hjust
grob$children[[1]]$vjust <- 1 - grob$children[[1]]$vjust
grob$children[[1]]$x <- unit(1, "npc") - grob$children[[1]]$x
grob
}
j = 1
k = 0
for(i in 1:npanel){
if ((i %% ncol == 0) || (i == npanel)){
k = k + 1
index <- which(g2$layout$name == "axis_l-1") # Which grob
yaxis <- g2$grobs[[index]] # Extract the grob
ticks <- yaxis$children[[2]]
ticks$widths <- rev(ticks$widths)
ticks$grobs <- rev(ticks$grobs)
ticks$grobs[[1]]$x <- ticks$grobs[[1]]$x - unit(1, "npc")
ticks$grobs[[2]] <- hinvert_title_grob(ticks$grobs[[2]])
yaxis$children[[2]] <- ticks
if (k == 1)#to ensure just once d secondary axisis printed
g <- gtable_add_cols(g,g2$widths[g2$layout[index,]$l],
max(pp$r[j:i]))
g <- gtable_add_grob(g,yaxis,max(pp$t[j:i]),max(pp$r[j:i])+1,
max(pp$b[j:i])
, max(pp$r[j:i]) + 1, clip = "off", name = "2ndaxis")
j = i + 1
}
}
# inserts the label for 2nd y-axis
loc_1st_yaxis_label <- c(subset(g$layout, grepl("ylab", g$layout$name), se
= t:r))
loc_2nd_yaxis_max_r <- c(subset(g$layout, grepl("2ndaxis", g$layout$name),
se = t:r))
zz <- max(loc_2nd_yaxis_max_r$r)+1
loc_1st_yaxis_label$l <- zz
loc_1st_yaxis_label$r <- zz
index <- which(g2$layout$name == "ylab")
ylab <- g2$grobs[[index]] # Extract that grob
ylab <- hinvert_title_grob(ylab)
ylab$children[[1]]$rot <- ylab$children[[1]]$rot + 180
g <- gtable_add_grob(g, ylab, loc_1st_yaxis_label$t, loc_1st_yaxis_label$l
, loc_1st_yaxis_label$b, loc_1st_yaxis_label$r
, clip = "off", name = "2ndylab")
grid.draw(g)
@Sandy 这是代码 its output
唯一的问题是，在最后一行中，次要 y 轴标签位于 panels.I 内，试图解决此问题但无法
您的 gtable_add_cols() 和 gtable_add_grob() 命令有问题。我在下面添加了评论。
更新到 ggplot2 v2.2.0
library(ggplot2)
library(gtable)
library(grid)
library(data.table)
library(scales)
diamonds$cut <- sample(letters[1:4], nrow(diamonds), replace = TRUE)
dt.diamonds <- as.data.table(diamonds)
d1 <- dt.diamonds[,list(revenue = sum(price),
stones = length(price)),
by=c("clarity", "cut")]
setkey(d1, clarity, cut)
# The facet_wrap plots
p1 <- ggplot(d1, aes(x = clarity, y = revenue, fill = cut)) +
geom_bar(stat = "identity") +
labs(x = "clarity", y = "revenue") +
facet_wrap( ~ cut, nrow = 2) +
scale_y_continuous(labels = dollar, expand = c(0, 0)) +
theme(axis.text.x = element_text(angle = 90, hjust = 1),
axis.text.y = element_text(colour = "#4B92DB"),
legend.position = "bottom")
p2 <- ggplot(d1, aes(x = clarity, y = stones, colour = "red")) +
geom_point(size = 4) +
labs(x = "", y = "number of stones") + expand_limits(y = 0) +
scale_y_continuous(labels = comma, expand = c(0, 0)) +
scale_colour_manual(name = '', values = c("red", "green"),
labels =c("Number of Stones")) +
facet_wrap( ~ cut, nrow = 2) +
theme(axis.text.y = element_text(colour = "red")) +
theme(panel.background = element_rect(fill = NA),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
panel.border = element_rect(fill = NA, colour = "grey50"),
legend.position = "bottom")
# Get the ggplot grobs
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
# Grab the panels from g2 and overlay them onto the panels of g1
pp <- c(subset(g1$layout, grepl("panel", g1$layout$name), select = t:r))
g <- gtable_add_grob(g1, g2$grobs[grepl("panel", g1$layout$name)],
pp$t, pp$l, pp$b, pp$l)
# Function to invert labels
hinvert_title_grob <- function(grob){
widths <- grob$widths
grob$widths[1] <- widths[3]
grob$widths[3] <- widths[1]
grob$vp[[1]]$layout$widths[1] <- widths[3]
grob$vp[[1]]$layout$widths[3] <- widths[1]
grob$children[[1]]$hjust <- 1 - grob$children[[1]]$hjust
grob$children[[1]]$vjust <- 1 - grob$children[[1]]$vjust
grob$children[[1]]$x <- unit(1, "npc") - grob$children[[1]]$x
grob
}
# Get the y label from g2, and invert it
index <- which(g2$layout$name == "ylab-l")
ylab <- g2$grobs[[index]] # Extract that grob
ylab <- hinvert_title_grob(ylab)
# Put the y label into g, to the right of the right-most panel
# Note: Only one column and one y label
g <- gtable_add_cols(g, g2$widths[g2$layout[index, ]$l], pos = max(pp$r))
g <-gtable_add_grob(g,ylab, t = min(pp$t), l = max(pp$r)+1,
b = max(pp$b), r = max(pp$r)+1,
clip = "off", name = "ylab-r")
# Get the y axis from g2, reverse the tick marks and the tick mark labels,
# and invert the tick mark labels
index <- which(g2$layout$name == "axis-l-1-1") # Which grob
yaxis <- g2$grobs[[index]] # Extract the grob
ticks <- yaxis$children[[2]]
ticks$widths <- rev(ticks$widths)
ticks$grobs <- rev(ticks$grobs)
plot_theme <- function(p) {
plyr::defaults(p$theme, theme_get())
}
tml <- plot_theme(p1)$axis.ticks.length # Tick mark length
ticks$grobs[[1]]$x <- ticks$grobs[[1]]$x - unit(1, "npc") + tml
ticks$grobs[[2]] <- hinvert_title_grob(ticks$grobs[[2]])
yaxis$children[[2]] <- ticks
# Put the y axis into g, to the right of the right-most panel
# Note: Only one column, but two y axes - one for each row of the facet_wrap plot
g <- gtable_add_cols(g, g2$widths[g2$layout[index, ]$l], pos = max(pp$r))
nrows = length(unique(pp$t)) # Number of rows
g <- gtable_add_grob(g, rep(list(yaxis), nrows),
t = unique(pp$t), l = max(pp$r)+1,
b = unique(pp$b), r = max(pp$r)+1,
clip = "off", name = paste0("axis-r-", 1:nrows))
# Get the legends
leg1 <- g1$grobs[[which(g1$layout$name == "guide-box")]]
leg2 <- g2$grobs[[which(g2$layout$name == "guide-box")]]
# Combine the legends
g$grobs[[which(g$layout$name == "guide-box")]] <-
gtable:::cbind_gtable(leg1, leg2, "first")
grid.newpage()
grid.draw(g)
SO 不是教程站点，这可能会招致其他 SO 用户的愤怒，但评论太多了。
仅使用一个绘图面板绘制图表（即，无分面），
library(ggplot2)
p <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point()
获取 ggplot grob。
g <- ggplotGrob(p)
探索剧情：
1) gtable_show_layout() 给出地块的 gtable 布局图。中间的大space是plot panel的位置。面板左侧和下方的列包含 y 轴和 x 轴。整个地块周围有一个边距。索引给出了数组中每个单元格的位置。请注意，例如，面板位于第四列的第三行。
gtable_show_layout(g)
2) 布局数据框。 g$layout returns 一个数据框，其中包含图中包含的 grobs 的名称以及它们在 gtable 中的位置：t、l、b 和 r（代表顶部、左侧、右侧和底部).请注意，例如，面板位于 t=3、l=4、b=3、r=4。这与上面从图中获得的面板位置相同。
g$layout
3) 布局图试图给出行和列的高度和宽度，但它们往往会重叠。相反，请使用 g$widths 和 g$heights。 1null 宽度和高度是绘图面板的宽度和高度。请注意，1null 是第 3 个高度和第 4 个宽度 - 又是 3 和 4。
现在画一个 facet_wrap 和一个 facet_grid 图。
p1 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_wrap(~ carb, nrow = 1)
p2 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_grid(. ~ carb)
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
这两个图看起来一样，但它们的 gtables 不同。此外，组件名称不同。
通常获取包含常见类型 grob 索引（即 t、l、b 和 r）的布局数据帧的子集很方便；说所有面板。
pp1 <- subset(g1$layout, grepl("panel", g1$layout$name), select = t:r)
pp2 <- subset(g2$layout, grepl("panel", g2$layout$name), select = t:r)
请注意，例如所有面板都在第 4 行（pp1$t、pp2$t）。
pp1$r 指的是包含绘图面板的列；
pp1$r + 1 指的是面板右侧的列；
max(pp1$r) 指的是包含面板的最右边的列；
max(pp1$r) + 1 指的是包含面板的最右列右侧的列；
等等。
最后画一个多行的facet_wrap图。
p3 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_wrap(~ carb, nrow = 2)
g3 <- ggplotGrob(p3)
像以前一样探索绘图，但还对布局数据框进行子集化以包含面板的索引。
pp3 <- subset(g3$layout, grepl("panel", g3$layout$name), select = t:r)
如您所料，pp3 告诉您绘图面板位于三列（4、7 和 10）和两行（4 和 8）中。
在向 gtable 添加行或列以及向 gtable 添加 grob 时使用这些索引。使用 ?gtable_add_rows 和 gtable_add_grob 检查这些命令。
此外，学习一些 grid，尤其是如何构造 grobs，以及单位的使用（一些资源在 SO 上的 r-grid 标签中给出。
我正在使用 facet_wrap 并且还能够绘制次要 y 轴。然而，标签并没有绘制在轴附近，而是绘制得很远。我意识到如果我了解如何操纵 grobs 的 gtable (t,b,l,r) 的坐标系，这一切都会得到解决。有人可以解释他们实际描述的方式和内容 - t:r = c(4,8,4,4) 是什么意思。
辅助 yaxis 与 ggplot 有很多链接，但是当 nrow/ncol 大于 1 时，它们会失败。所以请教我网格几何和 grobs 位置管理的基础知识。
编辑：代码
this is the final code written by me :
library(ggplot2)
library(gtable)
library(grid)
library(data.table)
library(scales)
# Data
diamonds$cut <- sample(letters[1:13], nrow(diamonds), replace = TRUE)
dt.diamonds <- as.data.table(diamonds)
d1 <- dt.diamonds[,list(revenue = sum(price),
stones = length(price)),
by=c("clarity", "cut")]
setkey(d1, clarity, cut)
# The facet_wrap plots
p1 <- ggplot(d1, aes(x = clarity, y = revenue, fill = cut)) +
geom_bar(stat = "identity") +
labs(x = "clarity", y = "revenue") +
facet_wrap( ~ cut) +
scale_y_continuous(labels = dollar, expand = c(0, 0)) +
theme(axis.text.x = element_text(angle = 90, hjust = 1),
axis.text.y = element_text(colour = "#4B92DB"),
legend.position = "bottom")
p2 <- ggplot(d1, aes(x = clarity, y = stones, colour = "red")) +
geom_point(size = 4) +
labs(x = "", y = "number of stones") + expand_limits(y = 0) +
scale_y_continuous(labels = comma, expand = c(0, 0)) +
scale_colour_manual(name = '', values = c("red", "green"),
labels = c("Number of Stones"))+
facet_wrap( ~ cut) +
theme(axis.text.y = element_text(colour = "red")) +
theme(panel.background = element_rect(fill = NA),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
panel.border = element_rect(fill = NA, colour = "grey50"),
legend.position = "bottom")
# Get the ggplot grobs
xx <- ggplot_build(p1)
g1 <- ggplot_gtable(xx)
yy <- ggplot_build(p2)
g2 <- ggplot_gtable(yy)
nrow = length(unique(xx$panel$layout$ROW))
ncol = length(unique(xx$panel$layout$COL))
npanel = length(xx$panel$layout$PANEL)
pp <- c(subset(g1$layout, grepl("panel", g1$layout$name), se = t:r))
g <- gtable_add_grob(g1, g2$grobs[grepl("panel", g1$layout$name)],
pp$t, pp$l, pp$b, pp$l)
hinvert_title_grob <- function(grob){
widths <- grob$widths
grob$widths[1] <- widths[3]
grob$widths[3] <- widths[1]
grob$vp[[1]]$layout$widths[1] <- widths[3]
grob$vp[[1]]$layout$widths[3] <- widths[1]
grob$children[[1]]$hjust <- 1 - grob$children[[1]]$hjust
grob$children[[1]]$vjust <- 1 - grob$children[[1]]$vjust
grob$children[[1]]$x <- unit(1, "npc") - grob$children[[1]]$x
grob
}
j = 1
k = 0
for(i in 1:npanel){
if ((i %% ncol == 0) || (i == npanel)){
k = k + 1
index <- which(g2$layout$name == "axis_l-1") # Which grob
yaxis <- g2$grobs[[index]] # Extract the grob
ticks <- yaxis$children[[2]]
ticks$widths <- rev(ticks$widths)
ticks$grobs <- rev(ticks$grobs)
ticks$grobs[[1]]$x <- ticks$grobs[[1]]$x - unit(1, "npc")
ticks$grobs[[2]] <- hinvert_title_grob(ticks$grobs[[2]])
yaxis$children[[2]] <- ticks
if (k == 1)#to ensure just once d secondary axisis printed
g <- gtable_add_cols(g,g2$widths[g2$layout[index,]$l],
max(pp$r[j:i]))
g <- gtable_add_grob(g,yaxis,max(pp$t[j:i]),max(pp$r[j:i])+1,
max(pp$b[j:i])
, max(pp$r[j:i]) + 1, clip = "off", name = "2ndaxis")
j = i + 1
}
}
# inserts the label for 2nd y-axis
loc_1st_yaxis_label <- c(subset(g$layout, grepl("ylab", g$layout$name), se
= t:r))
loc_2nd_yaxis_max_r <- c(subset(g$layout, grepl("2ndaxis", g$layout$name),
se = t:r))
zz <- max(loc_2nd_yaxis_max_r$r)+1
loc_1st_yaxis_label$l <- zz
loc_1st_yaxis_label$r <- zz
index <- which(g2$layout$name == "ylab")
ylab <- g2$grobs[[index]] # Extract that grob
ylab <- hinvert_title_grob(ylab)
ylab$children[[1]]$rot <- ylab$children[[1]]$rot + 180
g <- gtable_add_grob(g, ylab, loc_1st_yaxis_label$t, loc_1st_yaxis_label$l
, loc_1st_yaxis_label$b, loc_1st_yaxis_label$r
, clip = "off", name = "2ndylab")
grid.draw(g)
@Sandy 这是代码 its output
唯一的问题是，在最后一行中，次要 y 轴标签位于 panels.I 内，试图解决此问题但无法
您的 gtable_add_cols() 和 gtable_add_grob() 命令有问题。我在下面添加了评论。
更新到 ggplot2 v2.2.0
library(ggplot2)
library(gtable)
library(grid)
library(data.table)
library(scales)
diamonds$cut <- sample(letters[1:4], nrow(diamonds), replace = TRUE)
dt.diamonds <- as.data.table(diamonds)
d1 <- dt.diamonds[,list(revenue = sum(price),
stones = length(price)),
by=c("clarity", "cut")]
setkey(d1, clarity, cut)
# The facet_wrap plots
p1 <- ggplot(d1, aes(x = clarity, y = revenue, fill = cut)) +
geom_bar(stat = "identity") +
labs(x = "clarity", y = "revenue") +
facet_wrap( ~ cut, nrow = 2) +
scale_y_continuous(labels = dollar, expand = c(0, 0)) +
theme(axis.text.x = element_text(angle = 90, hjust = 1),
axis.text.y = element_text(colour = "#4B92DB"),
legend.position = "bottom")
p2 <- ggplot(d1, aes(x = clarity, y = stones, colour = "red")) +
geom_point(size = 4) +
labs(x = "", y = "number of stones") + expand_limits(y = 0) +
scale_y_continuous(labels = comma, expand = c(0, 0)) +
scale_colour_manual(name = '', values = c("red", "green"),
labels =c("Number of Stones")) +
facet_wrap( ~ cut, nrow = 2) +
theme(axis.text.y = element_text(colour = "red")) +
theme(panel.background = element_rect(fill = NA),
panel.grid.major = element_blank(),
panel.grid.minor = element_blank(),
panel.border = element_rect(fill = NA, colour = "grey50"),
legend.position = "bottom")
# Get the ggplot grobs
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
# Grab the panels from g2 and overlay them onto the panels of g1
pp <- c(subset(g1$layout, grepl("panel", g1$layout$name), select = t:r))
g <- gtable_add_grob(g1, g2$grobs[grepl("panel", g1$layout$name)],
pp$t, pp$l, pp$b, pp$l)
# Function to invert labels
hinvert_title_grob <- function(grob){
widths <- grob$widths
grob$widths[1] <- widths[3]
grob$widths[3] <- widths[1]
grob$vp[[1]]$layout$widths[1] <- widths[3]
grob$vp[[1]]$layout$widths[3] <- widths[1]
grob$children[[1]]$hjust <- 1 - grob$children[[1]]$hjust
grob$children[[1]]$vjust <- 1 - grob$children[[1]]$vjust
grob$children[[1]]$x <- unit(1, "npc") - grob$children[[1]]$x
grob
}
# Get the y label from g2, and invert it
index <- which(g2$layout$name == "ylab-l")
ylab <- g2$grobs[[index]] # Extract that grob
ylab <- hinvert_title_grob(ylab)
# Put the y label into g, to the right of the right-most panel
# Note: Only one column and one y label
g <- gtable_add_cols(g, g2$widths[g2$layout[index, ]$l], pos = max(pp$r))
g <-gtable_add_grob(g,ylab, t = min(pp$t), l = max(pp$r)+1,
b = max(pp$b), r = max(pp$r)+1,
clip = "off", name = "ylab-r")
# Get the y axis from g2, reverse the tick marks and the tick mark labels,
# and invert the tick mark labels
index <- which(g2$layout$name == "axis-l-1-1") # Which grob
yaxis <- g2$grobs[[index]] # Extract the grob
ticks <- yaxis$children[[2]]
ticks$widths <- rev(ticks$widths)
ticks$grobs <- rev(ticks$grobs)
plot_theme <- function(p) {
plyr::defaults(p$theme, theme_get())
}
tml <- plot_theme(p1)$axis.ticks.length # Tick mark length
ticks$grobs[[1]]$x <- ticks$grobs[[1]]$x - unit(1, "npc") + tml
ticks$grobs[[2]] <- hinvert_title_grob(ticks$grobs[[2]])
yaxis$children[[2]] <- ticks
# Put the y axis into g, to the right of the right-most panel
# Note: Only one column, but two y axes - one for each row of the facet_wrap plot
g <- gtable_add_cols(g, g2$widths[g2$layout[index, ]$l], pos = max(pp$r))
nrows = length(unique(pp$t)) # Number of rows
g <- gtable_add_grob(g, rep(list(yaxis), nrows),
t = unique(pp$t), l = max(pp$r)+1,
b = unique(pp$b), r = max(pp$r)+1,
clip = "off", name = paste0("axis-r-", 1:nrows))
# Get the legends
leg1 <- g1$grobs[[which(g1$layout$name == "guide-box")]]
leg2 <- g2$grobs[[which(g2$layout$name == "guide-box")]]
# Combine the legends
g$grobs[[which(g$layout$name == "guide-box")]] <-
gtable:::cbind_gtable(leg1, leg2, "first")
grid.newpage()
grid.draw(g)
SO 不是教程站点，这可能会招致其他 SO 用户的愤怒，但评论太多了。
仅使用一个绘图面板绘制图表（即，无分面），
library(ggplot2)
p <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point()
获取 ggplot grob。
g <- ggplotGrob(p)
探索剧情：
1) gtable_show_layout() 给出地块的 gtable 布局图。中间的大space是plot panel的位置。面板左侧和下方的列包含 y 轴和 x 轴。整个地块周围有一个边距。索引给出了数组中每个单元格的位置。请注意，例如，面板位于第四列的第三行。
gtable_show_layout(g)
2) 布局数据框。 g$layout returns 一个数据框，其中包含图中包含的 grobs 的名称以及它们在 gtable 中的位置：t、l、b 和 r（代表顶部、左侧、右侧和底部).请注意，例如，面板位于 t=3、l=4、b=3、r=4。这与上面从图中获得的面板位置相同。
g$layout
3) 布局图试图给出行和列的高度和宽度，但它们往往会重叠。相反，请使用 g$widths 和 g$heights。 1null 宽度和高度是绘图面板的宽度和高度。请注意，1null 是第 3 个高度和第 4 个宽度 - 又是 3 和 4。
现在画一个 facet_wrap 和一个 facet_grid 图。
p1 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_wrap(~ carb, nrow = 1)
p2 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_grid(. ~ carb)
g1 <- ggplotGrob(p1)
g2 <- ggplotGrob(p2)
这两个图看起来一样，但它们的 gtables 不同。此外，组件名称不同。
通常获取包含常见类型 grob 索引（即 t、l、b 和 r）的布局数据帧的子集很方便；说所有面板。
pp1 <- subset(g1$layout, grepl("panel", g1$layout$name), select = t:r)
pp2 <- subset(g2$layout, grepl("panel", g2$layout$name), select = t:r)
请注意，例如所有面板都在第 4 行（pp1$t、pp2$t）。
pp1$r 指的是包含绘图面板的列；
pp1$r + 1 指的是面板右侧的列；
max(pp1$r) 指的是包含面板的最右边的列；
max(pp1$r) + 1 指的是包含面板的最右列右侧的列；
等等。
最后画一个多行的facet_wrap图。
p3 <- ggplot(mtcars, aes(x = mpg, y = disp)) + geom_point() +
facet_wrap(~ carb, nrow = 2)
g3 <- ggplotGrob(p3)
像以前一样探索绘图，但还对布局数据框进行子集化以包含面板的索引。
pp3 <- subset(g3$layout, grepl("panel", g3$layout$name), select = t:r)
如您所料，pp3 告诉您绘图面板位于三列（4、7 和 10）和两行（4 和 8）中。
在向 gtable 添加行或列以及向 gtable 添加 grob 时使用这些索引。使用 ?gtable_add_rows 和 gtable_add_grob 检查这些命令。
此外，学习一些 grid，尤其是如何构造 grobs，以及单位的使用（一些资源在 SO 上的 r-grid 标签中给出。