如何使R ggplot2中两幅图的绘图面板尺寸完全一致？

问题说明 #

使用ggplot2绘制两张垂直堆叠的图形时，底部带x轴标签和标题的图（图2）因x轴元素占用空间，导致其绘图面板尺寸小于顶部无x轴标签的图（图1），y轴被压缩。需要让两张图的绘图面板完全对齐，同时保留底部图的x轴标签作为整体的x轴说明。

现有代码 #

图1（无x轴标签和标题） #

plot1 <- ggplot(data, aes(x=pred_1, y="response", fill=pred_2)) +
    geom_violin(width=0.7) +
    geom_jitter(position=position_jitter(width=0.05)) +
    ggtitle("") +
    ylab("response") +
    xlab("") + # 无x轴标题
    ylim(0,3300) +
    scale_x_discrete(labels=c("level1", "level2")) +
    scale_fill_manual(values=c("red3","green4")) +
    geom_point(data=average_data, aes(x=pred_1, y="response"),
               position=position_dodge(width=0.7), size=4, shape=22, 
               stroke=1, show.legend=FALSE, color="black", fill="grey") +
    geom_line(data=average_data, aes(x=pred_1,y="response",group=1), linewidth=1) +
    theme_minimal() +
    theme(axis.title.x = element_text(margin = margin(t = 5), size=12, face="bold"),
          axis.title.y = element_text(margin = margin(r = 5), size=12, face="bold"),
          axis.text.x = element_blank(), # 隐藏x轴刻度标签
          axis.text.y = element_text(size = 10),
          panel.border = element_rect(color = "black", fill = NA, size = 1),
          legend.position = "none",
          plot.title = element_text(hjust = 0.5))

图2（带x轴标签和标题） #

plot2 <- ggplot(data_2, aes(x=pred_1, y="response", fill=pred_2)) +
    geom_violin(width=0.7) +
    geom_jitter(position=position_jitter(width=0.05)) +
    ggtitle("") +
    ylab("response") +
    xlab("this is my x-axis title") +
    ylim(0,3300) +
    scale_x_discrete(labels=c("level1", "level2")) +
    scale_fill_manual(values=c("red3","green4")) +
    geom_point(data=average_data_2, aes(x=pred_1, y="response"),
               position=position_dodge(width=0.7), size=4, shape=22, 
               stroke=1, show.legend=FALSE, color="black", fill="grey") +
    geom_line(data=average_data_2, aes(x=pred_1,y="response",group=1), linewidth=1) +
    theme_minimal() +
    theme(axis.title.x = element_text(margin = margin(t = 5), size=12, face="bold"),
          axis.title.y = element_text(margin = margin(r = 5), size=12, face="bold"),
          axis.text.y = element_text(size = 10),
          panel.border = element_rect(color = "black", fill = NA, size = 1),
          legend.position = "none",
          plot.title = element_text(hjust = 0.5))

解决方案 #

方案1：使用patchwork包（推荐，对齐最省心） #

patchwork专门用于ggplot2图形组合，自动对齐面板，语法简洁：

# 安装并加载包
install.packages("patchwork")
library(patchwork)

# 垂直堆叠图形，plot1在上，plot2在下
combined_plot <- plot1 + plot2 + plot_layout(ncol = 1)

# 可选：调整margin减少图形间空隙，同时确保面板对齐
plot1 <- plot1 + theme(plot.margin = margin(t = 10, r = 10, b = 5, l = 10, "pt"))
plot2 <- plot2 + theme(plot.margin = margin(t = 5, r = 10, b = 10, l = 10, "pt"))
combined_plot <- plot1 / plot2 # 垂直堆叠的简写语法

方案2：用gtable统一面板高度 #

将ggplot对象转为gtable，手动设置两张图的面板高度一致：

library(gridExtra)
library(gtable)

# 转换为gtable对象
g1 <- ggplotGrob(plot1)
g2 <- ggplotGrob(plot2)

# 获取最大面板高度，统一设置给两张图
panel_height <- max(g1$heights[g1$layout$name == "panel"], 
                    g2$heights[g2$layout$name == "panel"])
g1$heights[g1$layout$name == "panel"] <- unit(panel_height, "null")
g2$heights[g2$layout$name == "panel"] <- unit(panel_height, "null")

# 组合图形
grid.arrange(g1, g2, ncol = 1)

方案3：手动调整图1的底部margin #

给图1底部预留与图2x轴标签+标题相同高度的空间，确保面板高度一致：

# 调整图1的底部margin，预留x轴元素空间
plot1 <- plot1 + theme(
  plot.margin = margin(t = 10, r = 10, b = 30, l = 10, "pt"), # 底部留30pt空间
  axis.title.x = element_blank()
)

# 调整图2的顶部margin减少空隙
plot2 <- plot2 + theme(plot.margin = margin(t = 0, r = 10, b = 10, l = 10, "pt"))

# 用ggpubr::ggarrange垂直对齐组合
library(ggpubr)
ggarrange(plot1, plot2, ncol = 1, align = "v")

内容的提问来源于stack exchange，提问作者RobH