使用coord_polar()和facet_wrap()绘制饼图时图例出现字母a的解决咨询

问题原因 #

你在绘图时同时给fill和color两个美学属性绑定了uniconnex变量，ggplot会自动合并这两个属性的图例。其中color属性的默认图例符号是文本（显示为小写字母"a"），和fill的填充矩形叠加后，就出现了图例框里的"a"。此外代码中还有几处冗余设置：

• 重复定义了8元素的uniconnex向量，但实际数据中该因子只有4个水平，导致标签重复；
• 冗余的group参数（fill已能实现分组）；
• labs()中多余的shape设置（未映射该属性）。

解决方案 #

方案1：移除不必要的color映射 #

如果不需要饼图边框，直接删除color相关的映射和设置，代码更简洁：

ggplot(df.connexuniv.location, aes(x="", y=percentage, fill=uniconnex)) + 
  geom_bar(width=1, stat="identity")  +
  coord_polar("y", start=0, direction=-1) + facet_wrap(~location) +
  scale_fill_manual(
    values=c("grey20","grey60", "grey90","grey99"),
    name = "Connection to university", 
    labels=levels(df.connexuniv.location$uniconnex)
  ) +
  labs(x=" ", y="percentage of audience") +
  theme(
    panel.border = element_rect(linetype = "solid", colour="black", linewidth=.5, fill = NA), 
    panel.background=element_rect(fill="grey97"), 
    strip.background = element_rect(colour = "black", fill = NA, size = .5), 
    axis.text.x = element_blank(),
    axis.ticks = element_blank(),
    panel.grid  = element_blank()
  ) +
  geom_text(aes(x = 1.7, label = paste0(round(percentage, digits=0), " %")), size=3, 
            position = position_stack(vjust = 0.5))

方案2：保留边框并调整图例符号 #

如果需要饼图边框，通过guides()隐藏color的文本符号，避免和fill图例叠加：

ggplot(df.connexuniv.location, aes(x="", y=percentage, fill=uniconnex, color=uniconnex)) + 
  geom_bar(width=1, stat="identity")  +
  coord_polar("y", start=0, direction=-1) + facet_wrap(~location) +
  scale_fill_manual(
    values=c("grey20","grey60", "grey90","grey99"),
    name = "Connection to university", 
    labels=levels(df.connexuniv.location$uniconnex)
  ) +
  scale_color_manual(
    values=rep("grey20",4),
    name = "Connection to university"
  ) +
  labs(x=" ", y="percentage of audience") +
  theme(
    panel.border = element_rect(linetype = "solid", colour="black", linewidth=.5, fill = NA), 
    panel.background=element_rect(fill="grey97"), 
    strip.background = element_rect(colour = "black", fill = NA, size = .5), 
    axis.text.x = element_blank(),
    axis.ticks = element_blank(),
    panel.grid  = element_blank()
  ) +
  geom_text(aes(x = 1.7, label = paste0(round(percentage, digits=0), " %")), size=3, 
            position = position_stack(vjust = 0.5)) +
  guides(color = guide_legend(override.aes = list(shape = NA)))

内容的提问来源于stack exchange，提问作者Hildrun Walter