R中ggplot绘制折线图报geom_path单组仅1个观测错误的原因及解决方法

• 你传入ggplot的x轴变量month是字符串类型，默认会被识别为离散分类变量，ggplot2默认规则是每个离散分类单独作为一组
• geom_line()要求同一个分组内至少有2个观测值才能生成连线，你的每个月份分类下只有1个value值，每个分组都没法生成连线，因此触发该报错
• 额外提示：直接用字符串类型的月份做x轴，还会出现月份按字母顺序排序、而非自然时间顺序的问题，也建议同步修复

方案1：直接添加group参数（最快解决） #

在aes映射中添加group = 1，强制所有观测值归为同一个分组，即可正常生成折线：

month =c("January","February","March","April","May","June",
                 "July",    "August" ,"September","October","November","December")
value = seq(1:12)
d = data.frame(month,value)
ggplot(data = d ,aes(x = month,y=value, group = 1)) + geom_line()

方案2：先将月份转为有序因子（同时解决排序问题） #

如果要保证x轴月份按自然时间顺序排列，先把month转为指定顺序的有序因子，再添加group参数绘图：

# 先把month转成有序因子，levels按1-12月的顺序设置
d$month <- factor(d$month, levels = c("January","February","March","April","May","June",
                                      "July","August","September","October","November","December"),
                  ordered = TRUE)
# 绘图时加group=1
ggplot(data = d ,aes(x = month,y=value, group = 1)) + geom_line()

内容的提问来源于stack exchange，提问作者Homer Jay Simpson