如何在R中保存坐标轴长度一致的两个ggplot图（不受轴文本位数影响）

问题核心是ggplot会根据元素（如y轴文本）自动调整布局，导致相同尺寸设置下，实际绘图面板大小不同。以下是几种独立保存时保证坐标轴长度一致的方法：

方法1：使用gtable固定面板宽度 #

通过gtable包手动调整绘图对象的面板宽度，确保两张图的绘图区域完全一致：

library(ggplot2)
library(gtable)
set.seed(123)

# 生成数据
x <- seq(0, 100, by = 1)
y1 <- x + rnorm(length(x), mean = 0, sd = 10)
y2 <- 200000 - x * 1000 + rnorm(length(x), mean = 0, sd = 10000)
data1 <- data.frame(x = x, y1 = y1)
data2 <- data.frame(x = x, y2 = y2)

# 创建绘图对象
plot1 <- ggplot(data1, aes(x = x, y = y1)) +
  geom_point() +
  geom_smooth(method = "lm", col = "blue") +
  theme_bw() +
  theme(aspect.ratio = 0.75)

plot2 <- ggplot(data2, aes(x = x, y = y2)) +
  geom_point() +
  geom_smooth(method = "lm", col = "red") +
  theme_bw() +
  theme(aspect.ratio = 0.75)

# 转换为gtable对象
gt1 <- ggplotGrob(plot1)
gt2 <- ggplotGrob(plot2)

# 获取两张图的面板宽度，取较大值作为统一宽度
panel_width <- max(gt1$widths[gt1$layout$name == "panel"], 
                   gt2$widths[gt2$layout$name == "panel"])

# 设置两张图的面板宽度为统一值
gt1$widths[gt1$layout$name == "panel"] <- panel_width
gt2$widths[gt2$layout$name == "panel"] <- panel_width

# 保存调整后的图
setwd("C:/[your path]")
tiff("plot1_fixed.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(gt1)
dev.off()

tiff("plot2_fixed.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(gt2)
dev.off()

方法2：用cowplot对齐后独立保存 #

cowplot的align_plots函数可以对齐多张图的面板，之后分别导出：

library(ggplot2)
library(cowplot)
set.seed(123)

# 生成数据和创建绘图对象
x <- seq(0, 100, by = 1)
y1 <- x + rnorm(length(x), mean = 0, sd = 10)
y2 <- 200000 - x * 1000 + rnorm(length(x), mean = 0, sd = 10000)
data1 <- data.frame(x = x, y1 = y1)
data2 <- data.frame(x = x, y2 = y2)

plot1 <- ggplot(data1, aes(x = x, y = y1)) +
  geom_point() +
  geom_smooth(method = "lm", col = "blue") +
  theme_bw() +
  theme(aspect.ratio = 0.75)

plot2 <- ggplot(data2, aes(x = x, y = y2)) +
  geom_point() +
  geom_smooth(method = "lm", col = "red") +
  theme_bw() +
  theme(aspect.ratio = 0.75)

# 对齐两张图（垂直方向对齐，保证x轴长度一致）
aligned_plots <- align_plots(plot1, plot2, align = "v", axis = "l")

# 分别保存对齐后的图
setwd("C:/[your path]")
tiff("plot1_aligned.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(aligned_plots[[1]])
dev.off()

tiff("plot2_aligned.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(aligned_plots[[2]])
dev.off()

方法3：手动设置左侧边距 #

通过theme()的plot.margin手动调整左边距，给较长的y轴文本预留足够空间，同时让两张图的绘图区域一致：

library(ggplot2)
set.seed(123)

# 生成数据和创建绘图对象
x <- seq(0, 100, by = 1)
y1 <- x + rnorm(length(x), mean = 0, sd = 10)
y2 <- 200000 - x * 1000 + rnorm(length(x), mean = 0, sd = 10000)
data1 <- data.frame(x = x, y1 = y1)
data2 <- data.frame(x = x, y2 = y2)

# 给两张图设置相同的左侧边距（根据较长y轴需求调整）
common_margin <- margin(5.5, 5.5, 5.5, 20, unit = "pt")

plot1 <- ggplot(data1, aes(x = x, y = y1)) +
  geom_point() +
  geom_smooth(method = "lm", col = "blue") +
  theme_bw() +
  theme(aspect.ratio = 0.75, plot.margin = common_margin)

plot2 <- ggplot(data2, aes(x = x, y = y2)) +
  geom_point() +
  geom_smooth(method = "lm", col = "red") +
  theme_bw() +
  theme(aspect.ratio = 0.75, plot.margin = common_margin)

# 保存图片
setwd("C:/[your path]")
tiff("plot1_margin.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(plot1)
dev.off()

tiff("plot2_margin.tiff", units="in", height=3, width=4, res=200)
grid::grid.draw(plot2)
dev.off()

方法1和方法2为自动对齐，准确性更高；方法3适合快速测试，需手动调整边距参数。

内容的提问来源于stack exchange，提问作者Giuseppe Petri