ggplot2中为柱状图+折线图加图例及解决数据转换报错

一、解决pivot_longer报错问题 #

报错核心原因是你传入了字符向量date，但pivot_longer的处理对象必须是数据框。正确用法是传入数据集df1，并明确指定需要转换的列：

# 先加载tidyverse包（pivot_longer属于tidyr模块，包含在tidyverse中）
library(tidyverse)

# 将宽数据转为长数据，指定要转换的列是cats、dogs、pigs
df2 <- pivot_longer(df1, 
                    cols = c(cats, dogs, pigs),  # 也可以用cols = -date，直接排除日期列
                    names_to = "animals", 
                    values_to = "count")

执行后会得到包含date、animals、count三列的长数据集，后续绘图会更简洁规范。

二、解决ggplot图例缺失和柱状图边框问题 #

方法1：基于长数据绘图（推荐方案） #

用转换后的长数据df2，可以自动生成正确图例，同时彻底解决边框问题：

library(ggplot2)
library(scales)  # 用于comma函数格式化y轴数字

p1 <- ggplot(df2, aes(x = date, y = count)) +
  # 绘制cats的柱状图，过滤出对应数据，colour=NA移除边框
  geom_bar(data = filter(df2, animals == "cats"), 
           stat = "identity", fill = "gray", colour = NA) +
  # 绘制dogs和pigs的折线+点，映射color和linetype到动物类型
  geom_line(data = filter(df2, animals != "cats"), 
            aes(color = animals, linetype = animals), size = 0.75, group = 1) +
  geom_point(data = filter(df2, animals != "cats"), 
             aes(color = animals), size = 1.5) +
  # 自定义颜色和线型
  scale_color_manual(values = c("dogs" = "red", "pigs" = "blue")) +
  scale_linetype_manual(values = c("dogs" = "solid", "pigs" = "dashed")) +
  # 轴标签与范围设置
  scale_y_continuous(labels = comma, limits = c(0, 550000), name = "number") +
  labs(x = "Year", color = "Animals", linetype = "Animals") +
  theme(axis.text.x = element_text(angle = 60, vjust = 0.5))

p1

方法2：基于原宽数据修改（适配原有代码） #

如果不想转换数据，直接修改原有代码也能解决问题：

1. 移除柱状图边框：把aes里的colour = "cats"删除，或者给geom_bar设置colour = NA，用fill指定柱子填充色；
2. 生成折线图例：将color放入aes映射中，配合scale_color_manual指定颜色，同时处理线型映射。

修改后的代码：

library(ggplot2)
library(scales)

p1 <- ggplot(data = df1, aes(x = date)) +
  # 柱状图：移除colour的aes映射，设置填充色并去掉边框
  geom_bar(aes(y = cats), stat = "identity", fill = "gray", colour = NA) +
  # 折线和点：将color放入aes，映射到对应动物名称
  geom_line(aes(y = dogs, color = "dogs", group = 1), size = 0.75) +
  geom_point(aes(y = dogs, color = "dogs"), size = 1.5) +
  geom_line(aes(y = pigs, color = "pigs", linetype = "pigs", group = 1), size = 0.75) +
  geom_point(aes(y = pigs, color = "pigs"), size = 1.5) +
  # 自定义颜色和线型，统一图例名称
  scale_color_manual(name = "Animals", values = c("dogs" = "red", "pigs" = "blue")) +
  scale_linetype_manual(name = "Animals", values = c("pigs" = "dashed")) +
  # 轴设置
  scale_y_continuous(labels = comma, limits = c(0, 550000), name = "number") +
  labs(x = "Year") +
  theme(axis.text.x = element_text(angle = 60, vjust = 0.5))

p1

内容的提问来源于stack exchange，提问作者KT_1