ggplot2修改x/y轴刻度标签后坐标轴消失问题解决方法

刻度消失的核心原因是数据类型和标度函数不匹配：你的time、plasma列都是数值型连续变量，强行调用scale_x_discrete()/scale_y_discrete()离散标度时，ggplot无法将连续数值映射到离散刻度规则上，就会出现轴刻度完全不显示的问题。你判断的方向是对的，应该改用连续标度函数实现效果。

以下代码可以高度贴合参考图的呈现效果，包括轴单位放在刻度末端、简洁的绘图样式：

library(ggplot2)

time <- c(0, 120, 0, 10, 18, 0, 
          20, 22, 0, 42, 48, 62,
          0, 21, 38)
ph <- as.factor(c(6, 6, 5.5, 5.5, 5.5, 5.1,
                  5.1, 5.1, 4.5, 4.5, 4.5, 4.5,
                  4.3, 4.3, 4.3))
plasma <- c(130, 130, 130, 119, 101,
            130, 100,92, 130,
            115, 108, 102, 130, 115,
            111)
lab6 <- c(" ", "pH 6.0", " ", " ", " ", " ",
          " ", " ", " ", " ", " ", " ",
          " ", " ", "")
lab5 <- c(" ", " ", " ", " ", "pH 5.5 ", " ",
          " ", "pH 5.1 ", " ", " ", " ", " ",
          " ", " ", "")
lab4 <- c(" ", " ", " ", " ", " ", " ",
          " ", " ", " ", " ", " ", "pH 4.5 ",
          " ", " ", "pH 4.3")

d <- data.frame(time, ph, plasma, lab6, lab5, lab4)

ggplot(d, aes(time, plasma)) +
  geom_line(aes(colour = ph), linewidth = 0.8) +
  geom_point(aes(colour = ph), size = 2) +
  geom_text(nudge_x = -5, 
            nudge_y = -2,
            aes(label = lab6, 
                colour = ph)) +
  geom_text(nudge_x = -7,
            nudge_y = 0,
            aes(label = lab5,
                colour = ph)) +
  geom_text(nudge_x = -8,
            nudge_y = 0,
            aes(label = lab4,
                colour = ph)) +
  # 用连续标度匹配数值型数据，单位直接作为末端刻度标签
  scale_x_continuous(name = NULL, 
                     breaks = c(0, 50, 100, 130), 
                     labels = c("0", "50", "100", "Hrs"),
                     limits = c(0, 130),
                     expand = c(0.01, 0)) +
  scale_y_continuous(name = NULL, 
                     breaks = c(60, 80, 100, 120, 130),
                     labels = c("60", "80", "100", "120", "me/l"),
                     limits = c(88, 135),
                     expand = c(0.01, 0)) +
  # 匹配参考图的简洁无网格样式
  theme_classic() +
  # 去掉多余的图例（参考图直接在线条旁标注pH值，不需要图例）
  theme(legend.position = "none")

• 替换离散标度为scale_x_continuous()/scale_y_continuous()，匹配连续数值的数据类型，从根源解决刻度消失问题
• 把默认轴标题设为NULL，直接将单位文本（Hrs/me/l）设置为轴末端刻度的标签，完美复现参考图里单位放在刻度端点的效果，不需要额外添加注释层
• 增加theme_classic()去掉默认背景网格，隐藏多余图例，调整轴范围和边距让元素布局更贴合参考图样式

内容的提问来源于stack exchange，提问作者asellus