如何在ggplot2散点图中绘制y值的上下边界线？

你可以通过stat_summary()函数直接对x分箱计算y的极值，再拟合平滑边界线，和你现有geom_smooth的均值趋势线风格保持一致，完整实现代码如下：

library(ggplot2)
suaon <- mtcars
pla <- ggplot(data = suaon, mapping = aes(x = mpg, y = wt))
# 原有散点层
pla <- pla + geom_point(mapping = aes(color = factor(cyl)), size = 1.4)
# 原有均值拟合线
pla <- pla + geom_smooth(span = 0.4, method = "loess", se = FALSE,color = "red", size = 0.8)
# 新增最大值上边界线
pla <- pla + stat_summary(fun = max, geom = "smooth", span = 0.4, method = "loess", se = FALSE, color = "darkgreen", size = 0.8)
# 新增最小值下边界线
pla <- pla + stat_summary(fun = min, geom = "smooth", span = 0.4, method = "loess", se = FALSE, color = "darkgreen", size = 0.8)
pla <- pla + labs(x = "MPG", y = "WT", color = "气缸数")
pla

补充说明：

• 代码中span和method参数和你原有均值拟合线参数一致，可根据需要调整平滑度
• 若需要非平滑的折线式边界，将stat_summary的geom参数改为"line"，删除method和span参数即可

内容的提问来源于stack exchange，提问作者Mehdi