如何为ggplot2分面图的每个子图单独设置ylim()？

方案1：自动适配各子图y轴范围 #

全局ylim()会强制所有子图使用同一y轴范围，只需给facet_grid添加scales = "free_y"参数，就能让每个子图根据自身数据自动匹配合适的y轴范围：

ggplot(data = Factors, 
       mapping = aes(x = as.Date(Date,"%m/%d/%Y"), y = Variable, group = 1)) +
  geom_line(group = 1) + 
  facet_grid(rows=vars(Group), scales = "free_y") +  # 核心修改：开启y轴自由缩放
  labs(x = "Date", y = "Variable") +
  scale_x_date(date_breaks = "week", date_labels = "%b %d")

如果要改成散点图，直接把geom_line(group = 1)替换为geom_point()即可：

ggplot(data = Factors, 
       mapping = aes(x = as.Date(Date,"%m/%d/%Y"), y = Variable, group = 1)) +
  geom_point() +  # 替换为散点图层
  facet_grid(rows=vars(Group), scales = "free_y") +
  labs(x = "Date", y = "Variable") +
  scale_x_date(date_breaks = "week", date_labels = "%b %d")

方案2：手动指定每个分组的精确y轴范围 #

如果需要严格按照你给出的范围（温度10-30、盐度20-40、叶绿素0-10）设置，先定义分组与y轴范围的映射表，再通过自定义函数指定每个子图的y轴限制：

1. 先创建分组映射：

# 按分组名称定义对应的y轴范围
y_range_map <- list(
  温度 = c(10, 30),
  盐度 = c(20, 40),
  叶绿素 = c(0, 10)
)

2. 修改绘图代码，手动绑定每个分组的y轴范围：

ggplot(data = Factors, 
       mapping = aes(x = as.Date(Date,"%m/%d/%Y"), y = Variable, group = 1)) +
  geom_line(group = 1) + 
  facet_grid(rows=vars(Group), scales = "free_y") +
  labs(x = "Date", y = "Variable") +
  scale_x_date(date_breaks = "week", date_labels = "%b %d") +
  scale_y_continuous(
    limits = function(x) {
      # 根据当前子图的数据匹配对应的分组范围
      current_group <- unique(Factors$Group[Factors$Variable >= min(x) & Factors$Variable <= max(x)])
      y_range_map[[current_group]]
    }
  )

同样，把geom_line换成geom_point就能得到指定y范围的散点分面图。

内容的提问来源于stack exchange，提问作者R-ookie