ggplot2中使用plot_grid组合图时图例不显示及报错的解决咨询

问题原因 #

报错提示Multiple components found; returning the first one说明get_legend()从legend_plot中识别到多个图例相关组件，导致提取的图例无效，最终组合图中无法正常显示。

修复方案 #

方案1：简化图例提取的空图逻辑 #

生成仅用于提取图例的空画布，清空绘图区域的冗余元素，避免干扰图例识别：

# 生成仅保留图例设置的空图
legend_plot <- ggplot(data_longGW, aes(x = Crop_cycleGW, y = kg_CO2_eq, fill = Category)) +
  geom_bar(stat = "identity", colour = "grey30") +
  scale_fill_manual(values = custom_colors) +
  theme_minimal() +
  theme(legend.position = "bottom",
        legend.title = element_blank(),
        legend.background = element_rect(fill = "white", size = 0.5, linetype = "solid", colour = "black")) +
  # 清空绘图区域元素，只保留图例
  theme(axis.title = element_blank(), axis.text = element_blank(), axis.ticks = element_blank(),
        panel.grid = element_blank())

legend <- get_legend(legend_plot)
combined_plot <- plot_grid(
  plot_grid(GW, TA, FE, ME, ET, FRS, ncol = 3),
  legend,
  ncol = 1,
  rel_heights = c(1, 0.1)
)
print(combined_plot)

方案2：子图统一控制图例显示 #

如果所有子图（GW、TA等）的填充映射规则一致，可让单个子图显示图例，其余子图隐藏，再组合：

# 隐藏所有子图的图例
GW <- GW + theme(legend.position = "none")
TA <- TA + theme(legend.position = "none")
FE <- FE + theme(legend.position = "none")
ME <- ME + theme(legend.position = "none")
ET <- ET + theme(legend.position = "none")

# 仅让最后一个子图保留图例，并调整样式
FRS <- FRS + theme(legend.position = "bottom",
                   legend.title = element_blank(),
                   legend.background = element_rect(fill = "white", size = 0.5, linetype = "solid", colour = "black"))

# 组合子图，将带图例的子图单独占一行
combined_plot <- plot_grid(
  plot_grid(GW, TA, FE, ME, ET, ncol = 3),
  # 清空该子图的绘图元素，只留图例
  plot_grid(FRS + theme(axis.title = element_blank(), axis.text = element_blank(), axis.ticks = element_blank(),
                        panel.grid = element_blank()), ncol=1),
  ncol = 1,
  rel_heights = c(1, 0.1)
)
print(combined_plot)

方案3：用patchwork包替代plot_grid（更简洁） #

patchwork对图例的管理更灵活，可直接为组合图添加统一图例：

library(patchwork)

# 隐藏所有子图的图例
GW <- GW + theme(legend.position = "none")
TA <- TA + theme(legend.position = "none")
FE <- FE + theme(legend.position = "none")
ME <- ME + theme(legend.position = "none")
ET <- ET + theme(legend.position = "none")
FRS <- FRS + theme(legend.position = "none")

# 组合子图并添加底部图例
combined_plot <- (GW + TA + FE + ME + ET + FRS) + 
  plot_layout(ncol = 3) +
  guide_area() +
  plot_layout(heights = c(1, 0.1)) +
  # 应用图例样式
  theme(legend.position = "bottom",
        legend.title = element_blank(),
        legend.background = element_rect(fill = "white", size = 0.5, linetype = "solid", colour = "black"))

print(combined_plot)

关键注意点 #

• 确保custom_colors的长度与Category的水平数完全匹配，避免颜色映射异常
• 提取图例时，尽量减少绘图区域的冗余元素，降低多组件识别冲突概率

内容的提问来源于stack exchange，提问作者Guido Evangelista