在R语言ggplot2的facet_wrap中设置变量自定义显示顺序

我有一个数据集，需要分组后用facet_wrap分面展示。目前分面功能正常，但填充变量（Name）按字母顺序排序，而非数据框中的原始顺序。常规绘图中可通过scale_x_discrete指定顺序，但搭配facet_wrap使用时出现如下错误：

Error in seq_len(n) : argument must be coercible to non-negative integer
In addition: Warning messages:
1: Removed 8 rows containing missing values (position_stack).
2: In max(f) : no non-missing arguments to max; returning -Inf

需求：左侧分面展示Jim、Bob，右侧展示Sue、Sally的顺序。

原代码示例 #

无scale_x_discrete的代码 #

library(ggplot2)
library(reshape2)

df_dtype <- data.frame(Name = c("Jim","Bob","Sue","Sally"),
                       Pre = c(150, 200, 325, 120),
                       Post = c(70,120,200,100) )

mdtype <- melt(df_dtype)

mdtype$group = ifelse(mdtype$Name %in% c("Jim", "Bob"),
                      "Group1", "Group2")

names = c("Jim","Bob","Sue","Sally")
p <- ggplot(mdtype, aes(x=variable, y=value, fill=Name)) + 
  geom_bar(position="stack", stat="identity") +
  ylab("Frequency") + xlab("") + ggtitle("Report Type") +
  theme(axis.ticks.x = element_blank(),axis.text.x = element_text(angle = 45, vjust = 1, hjust=1)) 
p + facet_wrap(~group) #+ scale_x_discrete(limits = names)

含scale_x_discrete的代码 #

library(ggplot2)
library(reshape2)

df_dtype <- data.frame(Name = c("Jim","Bob","Sue","Sally"),
                       Pre = c(150, 200, 325, 120),
                       Post = c(70,120,200,100) )

mdtype <- melt(df_dtype)

mdtype$group = ifelse(mdtype$Name %in% c("Jim", "Bob"),
                      "Group1", "Group2")

names = c("Jim","Bob","Sue","Sally")
p <- ggplot(mdtype, aes(x=variable, y=value, fill=Name)) + 
  geom_bar(position="stack", stat="identity") +
  ylab("Frequency") + xlab("") + ggtitle("Report Type") +
  theme(axis.ticks.x = element_blank(),axis.text.x = element_text(angle = 45, vjust = 1, hjust=1)) 
p + facet_wrap(~group) + scale_x_discrete(limits = names)

问题原因 #

错误根源是用错了scale参数：你要控制的是填充变量Name的顺序，但scale_x_discrete是用来设置x轴（此处为Pre/Post）的类别顺序。把Name的列表传给scale_x_discrete，会导致x轴找不到对应类别，进而触发报错。

解决方案 #

有两种可靠方法实现需求：

方法1：提前将Name转为有序因子 #

在数据预处理阶段，把Name设置为指定顺序的因子，ggplot会自动沿用这个顺序：

library(ggplot2)
library(reshape2)

df_dtype <- data.frame(Name = c("Jim","Bob","Sue","Sally"),
                       Pre = c(150, 200, 325, 120),
                       Post = c(70,120,200,100) )

# 自定义Name的顺序，转为有序因子
df_dtype$Name <- factor(df_dtype$Name, levels = c("Jim","Bob","Sue","Sally"), ordered = TRUE)

mdtype <- melt(df_dtype)

mdtype$group = ifelse(mdtype$Name %in% c("Jim", "Bob"),
                      "Group1", "Group2")

p <- ggplot(mdtype, aes(x=variable, y=value, fill=Name)) + 
  geom_bar(position="stack", stat="identity") +
  ylab("Frequency") + xlab("") + ggtitle("Report Type") +
  theme(axis.ticks.x = element_blank(),axis.text.x = element_text(angle = 45, vjust = 1, hjust=1)) 

p + facet_wrap(~group)

方法2：用scale_fill_discrete控制填充顺序 #

直接在绘图代码中用scale_fill_discrete指定Name的顺序，无需修改原始数据：

library(ggplot2)
library(reshape2)

df_dtype <- data.frame(Name = c("Jim","Bob","Sue","Sally"),
                       Pre = c(150, 200, 325, 120),
                       Post = c(70,120,200,100) )

mdtype <- melt(df_dtype)

mdtype$group = ifelse(mdtype$Name %in% c("Jim", "Bob"),
                      "Group1", "Group2")

name_order <- c("Jim","Bob","Sue","Sally")
p <- ggplot(mdtype, aes(x=variable, y=value, fill=Name)) + 
  geom_bar(position="stack", stat="identity") +
  ylab("Frequency") + xlab("") + ggtitle("Report Type") +
  theme(axis.ticks.x = element_blank(),axis.text.x = element_text(angle = 45, vjust = 1, hjust=1)) +
  scale_fill_discrete(limits = name_order) # 控制填充类别顺序

p + facet_wrap(~group)

两种方法都能让分面中的Name按需求顺序堆叠，且不会触发报错。

内容的提问来源于stack exchange，提问作者John Conor