如何在分组偏移箱线图中连接组内中位数？解决ggplot2报错

问题描述 #

使用interaction(method, var)作为分组绘制中位数散点时正常，但绘制组内中位数连线时出现报错：

geom_path: Each group consists of only one observation. Do you need to adjust
the group aesthetic?

当前效果：

期望效果：

报错原因 #

• 用interaction(method, var)分组时，每个分组是单个method与单个var的组合（比如A+X01、A+X02、B+X01、B+X02），每个分组仅对应1个中位数数据点。而geom_line要求同一个分组下至少有2个数据点才能生成连线，因此触发报错。
• 需求是同一method下，不同var之间的中位数连线，原分组逻辑与需求完全不符。

解决方法 #

将绘制连线的stat_summary中group参数改为method，让同一method下的不同var中位数点归为一组，满足连线的点数要求；同时保持position_dodge参数与箱线图、散点一致，确保所有元素对齐。

修正后的代码 #

dat <- 
    data.frame(method = c("A","A","A","A","A","A","A","A","A","A","B","B","B","B","B","B","B","B","B","B"),
               var = c("X01","X01","X01","X01","X01","X02","X02","X02","X02","X02","X01","X01","X01","X01","X01","X02","X02","X02","X02","X02"), 
               value = c(-1.1,0.3,0.3,-0.6,-0.2,0.7,1.2,0.2,0.3,1.8,0.5,1.1,-0.8,0.8,0.6,1.0,0.5,0.6,1.5,1.3))

ggplot(dat, aes(var, value)) +
    geom_boxplot(aes(fill = method), 
                 position = position_dodge(width = .9), 
                 outlier.color = NA) +
    stat_summary(
        fun = median,
        geom = "point",
        aes(group = interaction(method, var)),
        position = position_dodge(width = .9)
    ) +
    stat_summary(
        fun = median,
        geom = "line",
        aes(group = method),  # 此处修改为method
        position = position_dodge(width = .9)
    )

内容的提问来源于stack exchange，提问作者vitor