R语言ggplot2调整直方图堆叠条形的上下排列顺序

需求说明 #

绘制行驶距离相关直方图时，按剩余行驶距离将数据分为三类着色：

• <0km：未抵达目的地，对应数据中Group=-1
• =0km：抵达目的地，对应数据中Group=0
• >0km：超出目的地范围，对应数据中Group=+1

当前绘图中间0值附近的堆叠条形顺序不符合要求，目标堆叠规则为：

• 绿色条（=0km组）置于最底层，和其余区域同色条位置相邻
• 蓝色条（<0km组）位于中间层
• 红色条（>0km组）置于最顶层

最小可复现数据 #

df <- structure(list(excess_distance_km = c(87.5, 27.8, 18, 22.3, 35, 
120.6, -0.3, 0, 0, 24.6, -0.2, 2.9, 2.9, 8.7, 27.6, 32.9, 32.9, 
0, -0.2, -52.3, 54.4, 64.2, 64.2, 64.2, 50.2, 43.5, -13.5, -17.6, 
82.1, 82.1, 0, 74, 34.9, 35.2, 0, 2, -11.4, 0, 2.5, -52.3, 68.1, 
18, 0, 74.5, 29.6, 0.2, 18.9, 0, 0.7, 1.4), Group = c("+1", "+1", 
"+1", "+1", "+1", "+1", "-1", "0", "0", "+1", "-1", "+1", "+1", 
"+1", "+1", "+1", "+1", "0", "-1", "-1", "+1", "+1", "+1", "+1", 
"+1", "+1", "-1", "-1", "+1", "+1", "0", "+1", "+1", "+1", "0", 
"+1", "-1", "0", "+1", "-1", "+1", "+1", "0", "+1", "+1", "+1", 
"+1", "0", "+1", "+1")), row.names = c(NA, -50L), class = "data.frame")

当前使用代码 #

df %>%
  ggplot( aes(x = excess_distance_km)) +
    geom_histogram(aes(fill = Group), binwidth=5, alpha=0.9, colour = "grey25") +
    ggtitle("Excess Distance") +
    theme_classic() +
    theme(plot.title = element_text(size=15, hjust = 0.5),
          axis.title = element_text(size=12, hjust = 0.5),
          legend.text = element_text(size=12),
          legend.title = element_blank(),
          legend.position = "bottom"
    ) +
  guides(fill = guide_legend(nrow = 3)) 

当前运行效果 #

解决方法 #

ggplot2 中填充几何对象的堆叠顺序由映射到fill的变量的因子水平顺序决定：因子中排在最前的水平位于堆叠最底层（紧贴坐标轴），排在最后的水平位于堆叠最顶层。
原数据中Group是字符型，默认按字符串字典序排序，导致堆叠顺序不符合要求。只需提前将Group转为指定水平顺序的因子即可，修改后完整代码如下：

library(tidyverse)
df %>%
  # 手动指定因子水平，顺序为：最底层 -> 中间层 -> 最顶层
  mutate(Group = factor(Group, levels = c("0", "-1", "+1"))) %>%
  ggplot(aes(x = excess_distance_km)) +
    geom_histogram(aes(fill = Group), binwidth=5, alpha=0.9, colour = "grey25") +
    # 可选：手动指定颜色和标签，和三类语义完全匹配
    # scale_fill_manual(
    #   values = c("0" = "#2ca02c", "-1" = "#1f77b4", "+1" = "#d62728"),
    #   labels = c("0" = "=0km（抵达目的地）", "-1" = "<0km（未抵达目的地）", "+1" = ">0km（超出目的地范围）")
    # ) +
    ggtitle("Excess Distance") +
    theme_classic() +
    theme(plot.title = element_text(size=15, hjust = 0.5),
          axis.title = element_text(size=12, hjust = 0.5),
          legend.text = element_text(size=12),
          legend.title = element_blank(),
          legend.position = "bottom"
    ) +
  guides(fill = guide_legend(nrow = 3)) 

修改后绿色的抵达组会固定在堆叠最底层，和左右两侧的绿色条保持连续，中间为蓝色未抵达组，最上层为红色超出组，完全符合预期顺序。

内容的提问来源于stack exchange，提问作者ltong