如何在R的ggplot中创建双轴（柱状+折线）图表？

问题 #

我需要在R的ggplot中创建类似Excel的双轴图表，包含柱状图和折线图，两个Y轴的刻度范围不同。我已经分别写出了柱状图和折线图的代码，但尝试多种方法都无法将二者合并，请求帮助实现整合。

原柱状图代码：

inf_conc <- ggplot(data=data, aes(x=Day, y=inf)) +
 geom_bar(stat="identity", width=0.4, color="red3", fill="red3") +  
 ggtitle("Influent Microplastic Concentration 
 and Flow Rate") + 
  xlab("Day") + 
  ylab("Microplastic Concentration (MPs/L)") +
  scale_y_continuous(limits =c(0, 50), breaks = seq(0, 50, 5))

inf_conc + theme(axis.text = element_text(size = 20,  colour = "black"), 
             plot.title = element_text(size =25,  hjust = 0.5, 
             face = "bold"), axis.title = element_text(size = 20,  
             face = "bold", margin = 5))

原折线图代码：

inf_flow <- ggplot(data=data, aes(x=Day, y=flow, group = 1)) +
  geom_line(stat = "identity", colour ="blue4") +
  geom_point(colour ="blue4") +
   ylab("Inlet flow L/s")+
  xlab("Day")+
scale_y_continuous(limits=c(0,800), breaks = seq(0, 800, 100))

inf_flow + theme(axis.text = element_text(size = 20,  colour = "black"), 
     plot.title = element_text (size =25,  hjust = 0.5, face = "bold"), 
     axis.title = element_text(size = 20,  face = "bold", margin = 5))

解决方案 #

ggplot原生支持通过sec_axis()创建次坐标轴，核心是将折线图的Y轴数据映射到柱状图的Y轴范围，再通过刻度转换还原真实值。以下是整合后的完整代码：

library(ggplot2)

# 计算转换比例：将flow的0-800范围映射到inf的0-50范围
scale_factor <- 50 / 800

combined_plot <- ggplot(data = data, aes(x = Day)) +
  # 柱状图（主Y轴：左侧）
  geom_bar(aes(y = inf), stat = "identity", width = 0.4, color = "red3", fill = "red3") +
  # 折线图（数据先按比例缩放到主Y轴范围）
  geom_line(aes(y = flow * scale_factor), colour = "blue4", group = 1) +
  geom_point(aes(y = flow * scale_factor), colour = "blue4") +
  # 设置主、次Y轴
  scale_y_continuous(
    name = "Microplastic Concentration (MPs/L)",
    limits = c(0, 50),
    breaks = seq(0, 50, 5),
    # 次Y轴（右侧）：还原flow的真实刻度
    sec.axis = sec_axis(
      trans = ~ . / scale_factor,
      name = "Inlet flow L/s",
      limits = c(0, 800),
      breaks = seq(0, 800, 100)
    )
  ) +
  # 标题与轴标签
  ggtitle("Influent Microplastic Concentration and Flow Rate") +
  xlab("Day") +
  # 统一主题样式
  theme(
    axis.text = element_text(size = 20, colour = "black"),
    plot.title = element_text(size = 25, hjust = 0.5, face = "bold"),
    axis.title = element_text(size = 20, face = "bold", margin = margin(0, 20, 0, 0)),
    # 次Y轴文字与折线同色
    axis.title.y.right = element_text(color = "blue4"),
    axis.text.y.right = element_text(color = "blue4"),
    # 主Y轴文字与柱状图同色
    axis.title.y.left = element_text(color = "red3"),
    axis.text.y.left = element_text(color = "red3")
  )

print(combined_plot)

关键细节 #

1. 数据缩放：折线图的flow数据必须乘以scale_factor，将其数值范围压缩到柱状图的Y轴区间内，否则无法在同一坐标系中显示。
2. 刻度还原：sec_axis()的trans参数使用反向公式，将缩放后的值还原为flow的真实刻度，确保次Y轴显示正确数值。
3. 视觉区分：通过颜色匹配坐标轴与对应图表元素，让读者更容易区分两组数据。
4. 主题优化：合并原代码中重复的theme设置，避免冗余，同时调整边距让布局更美观。

内容的提问来源于stack exchange，提问作者D.dot