使用ggplot2的coord_polar()时如何在面板外添加文本？

问题描述 #

需要创建连续变量的环形图例图表，要在环形外侧（原x轴下方区域）添加方向文本，但使用annotate()或geom_text()配合coord_polar()时无法达到预期效果。

数据与原绘图代码 #

数据定义 #

# DATA
aspect_class = data.frame(from = c(0, seq(22.5, 337.5, by = 45)),
                          to =   c(seq(22.5, 337.5, by = 45), 360),
                          mid = seq(0, 360, by = 45),
                          class = c("North (0°-22.5°)", 
                                    "Northeast (22.5°-67.5°)",
                                    "East (67.5° - 112.5°)",
                                    "Southeast (112.5°-157.5°)",
                                    "South (157.5°-202.5°)",
                                    "Southwest (202.5°-247.5°)",
                                    "West (247.5°-292.5°)",
                                    "Northwest (292.5°-337.5°)",
                                    "North (337.5°-360°)"))

degrees = seq(0, 360, by = 1)
values = sin(degrees * pi / 180)
aspect_legend_df = data.frame(degrees, values)
compas = data.frame(degrees = seq(0,315,45),
                    direction = c("N", "NE", "E", "SE", "S", "SW", "W", "NW"))

原绘图代码 #

# PLOT
ggplot() +
  geom_tile(data = aspect_legend_df, 
            aes(x = degrees, y = 0, fill = values), 
            width = 1, height = 0.1) +
  scale_x_continuous(breaks = compas$degrees, 
                     labels = ~ paste0(.x, "\u00B0"),
                     name = "Aspect [°]") +
  scale_fill_viridis_c() +
  geom_vline(data = aspect_class[aspect_class$mid %in% c(0, 90, 180, 270),],
             aes(xintercept = mid), 
             color = "black", linewidth = 0.8) +
  geom_vline(data = aspect_class[!aspect_class$mid %in% c(0, 90, 180, 270, 360),],
             aes(xintercept = mid), 
             color = "black", linewidth = 0.5, linetype = "dotdash") +
  annotate(geom = "text", x = compas$degrees, y = 0.06, label = "") +    
  coord_polar(start = 0) + 
  theme_minimal() +
  theme(legend.position = "none",
        axis.title = element_blank(),
        axis.text.y = element_blank(),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank(),
        axis.text.x = element_text(size = 12, color = "black")) 

解决方案 #

问题核心是coord_polar()转换后，y轴代表环形的半径，原代码设置的y=0.06处于环形内部，且与原有刻度文本重叠。要将文本放在外侧，需调整y值到环形外部区域，并处理轴文本重叠问题。

修改后的绘图代码 #

ggplot() +
  # 绘制环形颜色带
  geom_tile(data = aspect_legend_df, 
            aes(x = degrees, y = 0, fill = values), 
            width = 1, height = 0.1) +
  # 隐藏原有x轴刻度文本，避免重叠
  scale_x_continuous(breaks = compas$degrees, 
                     labels = NULL,
                     name = "") +
  scale_fill_viridis_c() +
  # 添加分隔线
  geom_vline(data = aspect_class[aspect_class$mid %in% c(0, 90, 180, 270),],
             aes(xintercept = mid), 
             color = "black", linewidth = 0.8) +
  geom_vline(data = aspect_class[!aspect_class$mid %in% c(0, 90, 180, 270, 360),],
             aes(xintercept = mid), 
             color = "black", linewidth = 0.5, linetype = "dotdash") +
  # 添加外侧方向文本（y值设为0.2，确保在环形外部）
  geom_text(data = compas,
            aes(x = degrees, y = 0.2, label = direction), 
            inherit.aes = FALSE, 
            size = 5, fontface = "bold") +
  # 可选：添加外侧度数文本，与方向文本分层
  geom_text(data = compas,
            aes(x = degrees, y = 0.15, label = paste0(degrees, "\u00B0")), 
            inherit.aes = FALSE, 
            size = 3.5) +
  coord_polar(start = 0) + 
  theme_minimal() +
  theme(legend.position = "none",
        axis.title = element_blank(),
        axis.text.y = element_blank(),
        panel.grid.major = element_blank(),
        panel.grid.minor = element_blank(),
        axis.text.x = element_blank())

关键说明 #

• coord_polar()下，y值越大文本离中心越远，设置y=0.2可将方向文本固定在环形外侧
• 若需同时显示度数和方向，可设置不同y值实现分层展示
• 隐藏原有axis.text.x避免与自定义文本重叠

内容的提问来源于stack exchange，提问作者hopsasa.tralala