如何在不改变R绘图方向的前提下调整facet_grid分面顺序？

核心问题出在facet_grid的语法逻辑：该函数格式为**行变量 ~ 列变量**，左侧变量控制垂直方向（行）分面，右侧控制水平方向（列）分面。你之前把指定levels的factor放在了~右侧，导致分面变成水平排列，只需调整位置即可。

两种可行方案： #

方案1：提前将Timepoint转换为有序因子（推荐）

先修改数据集，把Timepoint转为指定顺序的因子，后续绘图时直接调用即可保持顺序，无需重复设置：

# 转换Timepoint为指定顺序的因子
LTheartbeats$Timepoint <- factor(
  LTheartbeats$Timepoint,
  levels = c("2hr", "8hr", "12hr", "18hr", "24hr"),
  ordered = TRUE
)

# 原绘图代码无需修改facet部分
ggplot(data = LTheartbeats, aes(x= factor(Condition), y = Heartbeats, fill = factor(Condition))) +
  geom_jitter(alpha = .1) +
  geom_violin(alpha = .75, trim = FALSE) +
  ylim(0,40)+
  facet_grid(Timepoint ~ .) +  # 直接使用已排序的因子
  scale_fill_discrete(name = "Treatment") + 
  labs(x = "\n Timepoint (Hours Post Exposure)", y = "Heartbeats (Per 10 Seconds) \n", 
       title = " \n") +
  theme(plot.title = element_text(hjust = 0.5),
        legend.title = element_text(colour="black", size=20,face="bold"), 
        legend.text = element_text(colour="black", size=15), 
        axis.text = element_text(size= 8),  
        axis.title.x = element_text(face="bold", colour="black", size = 20),
        axis.title.y = element_text(face="bold", colour="black", size = 20))

方案2：直接在facet_grid的行参数中指定顺序

如果不想修改原始数据集，可直接在facet_grid的行位置（~左侧）设置带levels的factor：

ggplot(data = LTheartbeats, aes(x= factor(Condition), y = Heartbeats, fill = factor(Condition))) +
  geom_jitter(alpha = .1) +
  geom_violin(alpha = .75, trim = FALSE) +
  ylim(0,40)+
  # 关键：将factor(Timepoint, levels=...)放在~左侧，控制垂直分面顺序
  facet_grid(factor(Timepoint, levels = c("2hr", "8hr", "12hr", "18hr", "24hr")) ~ .) + 
  scale_fill_discrete(name = "Treatment") + 
  labs(x = "\n Timepoint (Hours Post Exposure)", y = "Heartbeats (Per 10 Seconds) \n", 
       title = " \n") +
  theme(plot.title = element_text(hjust = 0.5),
        legend.title = element_text(colour="black", size=20,face="bold"), 
        legend.text = element_text(colour="black", size=15), 
        axis.text = element_text(size= 8),  
        axis.title.x = element_text(face="bold", colour="black", size = 20),
        axis.title.y = element_text(face="bold", colour="black", size = 20))

内容的提问来源于stack exchange，提问作者Michelle