添加手动图例致X轴扩展的ggplot技术问题排查

• X轴意外扩展的核心原因是geom_rect中设置的xmin=0和xmax=11超出了离散X轴的实际范围：
  ggplot会自动将离散型X轴（Site是分类变量）映射为连续数值，你数据里的8个分类对应1到8的区间。设置xmax=11会强制拉宽X轴范围到11，导致右侧出现大量空白扩展。
• 额外冗余操作：开头的group_by(Site)完全没必要，ggplot绘制箱线图时会自动按X轴分类分组；全局aes里的fill=cut(Phosphate,...)没有实际作用（你给geom_boxplot手动指定了填充色），还会造成不必要的映射冲突。

1. 调整geom_rect的xmin和xmax为离散X轴的实际边界：设xmin=0.5（第一个分类的左侧）、xmax=8.5（最后一个分类的右侧），让背景矩形刚好覆盖所有分类的X轴区域。
2. 移除冗余的group_by(Site)。
3. 移除全局aes中无用的fill映射。

修改后的代码：

df %>%
  ggplot(aes(x = Site, y = Phosphate)) +
  # 调整xmin/xmax匹配8个分类的X轴范围
  geom_rect(aes(ymin = 0, ymax = 0.069, xmin = 0.5, xmax = 8.5, fill = '0-0.069'), alpha=.1) +
  geom_rect(aes(ymin = 0.069, ymax = 0.173, xmin = 0.5, xmax = 8.5, fill = '0.069-0.173'), alpha=.1) +
  geom_rect(aes(ymin = 0.173, ymax = 1.003, xmin = 0.5, xmax = 8.5, fill = '0.173-1.003'), alpha=.1) +
  geom_rect(aes(ymin = 1.003, ymax = 12, xmin = 0.5, xmax = 8.5, fill = '1.003-12'), alpha=.1) +
  geom_boxplot(fill="darkolivegreen3") +
  scale_y_continuous(expand = c(0, 0), limits = c(0, 12)) +
  scale_x_discrete(limits = c("cell_1_t", "cell_1_b", "cell_2_t",
                              "cell_2_b", "cell_3_t", "cell_3_b",
                              "river_mun_us ", "river_mun_ds"),
                   labels = c("Cell 1 Top", "Cell 1 Bottom", "Cell 2 Top",
                              "Cell 2 Bottom", "Cell 3 Top", "Cell 3 Bottom",
                              "River Upstream", "River Downstream"),
                   expand = c(0, 0)) +
  annotate(geom = "text", x = 1:6, y = 1, fontface = "bold", label = "(15)") +
  annotate(geom = "text", x = 7, y = 3, fontface = "bold", label = "(15)") +
  annotate(geom = "text", x = 8, y = 5, fontface = "bold", label = "(15)") +
  ggtitle("River Mun ICW") +
  ylab(expression("Phosphate (mg P L"^-1*")")) +
  xlab(NULL) +
  scale_fill_manual(name = 'Water Quality',
                    breaks = c('0-0.069', '0.069-0.173', '0.173-1.003', '1.003-12'),
                    labels = c('Good', 'Moderate', 'Poor', 'Bad'),
                    values = c('bisque4', 'bisque3', 'bisque2', 'bisque')) +
  theme_bw() +
  theme(plot.title = element_text(hjust = 1, vjust = -10),
        axis.text.x = element_text(angle = 90, vjust = 1, hjust = 1, size = 12))+
  theme(legend.position = c(.9, .75),
        legend.background = element_rect(fill = "transparent"))

内容的提问来源于stack exchange，提问作者Erin