如何在最新版ggplot2中绘制混合sf POLYGON/MULTIPOLYGON几何对象

在最新版本的sf（v1.0.16）和ggplot2（v3.5.1）中，当聚合后的几何列同时包含POLYGON和MULTIPOLYGON类型时，使用geom_sf()绘制会抛出错误：! Not a matrix.。

library(data.table)
library(ggplot2)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1; sf_use_s2() is TRUE

# 查看包版本
packageVersion("sf")
#> [1] '1.0.16'
packageVersion("ggplot2")
#> [1] '3.5.1'

# 加载示例数据
nc <- sf::st_read(system.file("shape/nc.shp", package="sf"))
#> Reading layer `nc' from data source 
#>   `/mnt/rstudio_data/server-pro/c804893/R/x86_64-pc-linux-gnu-library/4.2/sf/shape/nc.shp' 
#>   using driver `ESRI Shapefile'
#> Simple feature collection with 100 features and 14 fields
#> Geometry type: MULTIPOLYGON
#> Dimension:     XY
#> Bounding box:  xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> Geodetic CRS:  NAD27

# 转换为data.table以便数据处理
dt <- data.table::as.data.table(nc)

# 按SID74聚合几何对象
dt_agg <- dt[, .(GEOMETRY = sf::st_union(geometry)), by = .(SID74)]

# 查看几何类型，存在POLYGON和MULTIPOLYGON混合
st_geometry_type(dt_agg$GEOMETRY)
#>  [1] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#>  [6] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#> [11] MULTIPOLYGON POLYGON      MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#> [16] MULTIPOLYGON POLYGON      MULTIPOLYGON POLYGON      POLYGON     
#> [21] MULTIPOLYGON POLYGON      POLYGON     
#> 18 Levels: GEOMETRY POINT LINESTRING POLYGON MULTIPOINT ... TRIANGLE

# 混合类型绘图报错
ggplot2::ggplot() +
  ggplot2::geom_sf(data = dt_agg, mapping = ggplot2::aes(geometry = GEOMETRY)) 
#> Error in `ggplot2::geom_sf()`:
#> ! Problem while converting geom to grob.
#> ℹ Error occurred in the 1st layer.
#> Caused by error:
#> ! Not a matrix.

# 仅MULTIPOLYGON可正常绘制
# ggplot2::ggplot() +
#   ggplot2::geom_sf(data = dt_agg[1:10], mapping = ggplot2::aes(geometry = GEOMETRY)) 

# 单个POLYGON也会报错
ggplot2::ggplot() +
  ggplot2::geom_sf(data = dt_agg[12], mapping = ggplot2::aes(geometry = GEOMETRY))
#> Error in `ggplot2::geom_sf()`:
#> ! Problem while converting geom to grob.
#> ℹ Error occurred in the 1st layer.
#> Caused by error:
#> ! Not a matrix.

问题根源是几何类型不统一，ggplot2::geom_sf()对混合的多边形类型处理存在兼容性问题。只需将所有几何对象统一转换为MULTIPOLYGON类型即可解决：

# 将几何列统一转换为MULTIPOLYGON
dt_agg$GEOMETRY <- sf::st_cast(dt_agg$GEOMETRY, "MULTIPOLYGON")

# 再次检查几何类型，已全部转为MULTIPOLYGON
st_geometry_type(dt_agg$GEOMETRY)
#>  [1] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#>  [6] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#> [11] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#> [16] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON
#> [21] MULTIPOLYGON MULTIPOLYGON MULTIPOLYGON

# 现在可以正常绘图
ggplot2::ggplot() +
  ggplot2::geom_sf(data = dt_agg, mapping = ggplot2::aes(geometry = GEOMETRY))

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value
#>  version  R version 4.2.0 (2022-04-22)
#>  os       Ubuntu 20.04.6 LTS
#>  system   x86_64, linux-gnu
#>  ui       X11
#>  language en_US:en
#>  collate  en_US.UTF-8
#>  ctype    en_US.UTF-8
#>  tz       Europe/Zurich
#>  date     2024-08-12
#>  pandoc   2.19.2 @ /usr/lib/rstudio-server/bin/quarto/bin/tools/ (via rmarkdown)

内容的提问来源于stack exchange，提问作者mnist