绘制sf对象时图例与scale_size_area不匹配问题

我有三个采用同一投影（EPSG:3005）的sf格式空间图层：两个线图层和一个点图层。我希望点的面积与其数值成比例，但点图层（数据框名为x）的图例无法匹配绘图效果——图例显示单一大小的方形符号，而绘图中是按比例变化的圆点。我尝试了以下两段代码，包括覆盖图例美学设置，但问题仍未解决，怀疑是否与sf对象的绘图机制有关。

尝试的代码1 #

ggplot() +
  geom_sf(data = track, color = "orange", lwd = 1.4) +          # line
  geom_sf(data = SZ_cropped, color = "grey40", lwd = 0.5) +     # line
  geom_sf(aes(size = total), data = x, color = "black", alpha = 0.7, shape = 21, fill = "purple") +      # dots
  scale_size_area(name = "Count") +                              # proportional area dots

尝试的代码2 #

ggplot() +
  geom_sf(data = track, color = "orange", lwd = 1.4) +          # line
  geom_sf(data = SZ_cropped, color = "grey40", lwd = 0.5) +     # line        
  geom_sf(aes(size = total), data = x, color = "black", alpha = 0.7, shape = 21, fill = "purple") +      # dots
  scale_size_area(name = "Count", max_size = 5) + # proportional area dots
  guides(size = guide_legend(override.aes = list(shape = 21, fill = "purple", color = "black"))) 

问题出在geom_sf的图例渲染逻辑，默认会采用空间对象的符号系统，导致手动设置的shape等参数在图例中未生效。以下两种方法可以解决：

方法1：指定key_glyph参数 #

在geom_sf中添加key_glyph = "point"，强制图例使用点型符号，覆盖sf默认的符号渲染：

ggplot() +
  geom_sf(data = track, color = "orange", lwd = 1.4) +          # 线图层1
  geom_sf(data = SZ_cropped, color = "grey40", lwd = 0.5) +     # 线图层2
  geom_sf(aes(size = total), data = x, color = "black", alpha = 0.7, shape = 21, fill = "purple",
          key_glyph = "point") +      # 关键：指定图例符号类型为点
  scale_size_area(name = "Count", max_size = 5) + 
  guides(size = guide_legend(override.aes = list(shape = 21, fill = "purple", color = "black")))

方法2：改用geom_point配合坐标提取 #

提取点图层的坐标后用普通geom_point绘制，绕开sf图例的特殊逻辑，更灵活控制图例：

# 提取点的坐标信息
x_coords <- st_coordinates(x)
x_df <- cbind(x, x_coords)

ggplot() +
  geom_sf(data = track, color = "orange", lwd = 1.4) +
  geom_sf(data = SZ_cropped, color = "grey40", lwd = 0.5) +
  geom_point(aes(X, Y, size = total), data = x_df, color = "black", alpha = 0.7, shape = 21, fill = "purple") +
  scale_size_area(name = "Count", max_size = 5)

• scale_size_area已经保证点的面积与total数值成比例，max_size可根据可视化需求调整最大点的尺寸
• 方法1更贴合sf对象的使用场景，方法2适合需要对图例进行更精细定制的情况

内容的提问来源于stack exchange，提问作者sbliss