ggplot2中如何实现折线按条件染色且不跳过中间不满足条件的点？

你当前的代码出现跨点连线的问题，是ggplot2默认分组逻辑导致的：geom_line()会把color映射对应的同一分组的所有点直接连接，不会考虑中间其他分组的点，因此所有判定为con_val>0的点会跳过中间不符合条件的点直接相连。

下面是两种可直接运行的解决方案：

方案1：使用ggforce包的geom_link2（最简实现） #

该函数支持根据每个点的属性给相邻线段赋值颜色，仅需少量修改即可实现需求：

library(tidyverse)
library(ggforce)

df_plot <- tibble(
  id = rep(1, times = 10),
  time = seq(from = 0, to = 25, length.out = 10), 
  y_val = c(seq(from = 50, to = 25, length.out = 5),
            seq(from = 25, to = 50, length.out = 5)),
  con_val = c(NA, NA, 1, NA, NA, NA, 5, NA, NA, 2)) %>%
  # 先明确条件列，把NA统一归为不满足条件
  mutate(cond = ifelse(is.na(con_val) | con_val <= 0, FALSE, TRUE))

ggplot(data = df_plot, aes(x = time, y = y_val, color = cond)) +
  geom_point() +
  # 用geom_link2代替geom_line，group=1保证所有点按顺序连接
  geom_link2(aes(group = 1)) +
  scale_colour_manual(values = setNames(c('red','gray'), c(TRUE, FALSE))) 

方案2：原生ggplot2实现（无需额外安装包） #

如果不想安装第三方包，可以先预处理数据，给每一段相邻线段分配颜色后用geom_segment绘制：

library(tidyverse)

df_plot <- tibble(
  id = rep(1, times = 10),
  time = seq(from = 0, to = 25, length.out = 10), 
  y_val = c(seq(from = 50, to = 25, length.out = 5),
            seq(from = 25, to = 50, length.out = 5)),
  con_val = c(NA, NA, 1, NA, NA, NA, 5, NA, NA, 2)) %>%
  mutate(
    cond = ifelse(is.na(con_val) | con_val <= 0, FALSE, TRUE),
    # 给相邻线段分配颜色：当前点和下一点都满足条件则为红色，否则为灰色
    seg_cond = ifelse(cond & lead(cond, default = last(cond)), TRUE, FALSE)
  )

ggplot(data = df_plot, aes(x = time, y = y_val)) +
  geom_point(aes(color = cond)) +
  # 逐段绘制线段
  geom_segment(aes(xend = lead(time), yend = lead(y_val), color = seg_cond)) +
  scale_colour_manual(values = setNames(c('red','gray'), c(TRUE, FALSE)))

如果需要线段颜色无过渡、严格按分界显示，优先选择方案2即可。

内容的提问来源于stack exchange，提问作者jsgraydon