R sf ggplot标记ID中列出的多个多边形

Question

假设我要绘制和标记多边形（数据框中的每一行一个多边形），我可以执行以下操作：

library(sf)
library(ggplot2)
nc <- sf::st_read(system.file("shape/nc.shp", package = "sf"), quiet = TRUE)
nc_3857 <- sf::st_transform(nc, 3857)
ggplot(nc_3857[1:3, ]) +
  geom_sf(aes(fill = AREA)) +
  geom_sf_label(aes(label = NAME))

这里的NAME仅指geometry变量中工作正常的一个多边形点列表。

但是，如果我的geometry变量代表每行包含多个多边形的列表，我将如何标记它们。我想要一个通用版本，可以解释不同长度的列表。例如，查看此df：

df <- structure(list(id= structure(1:2, .Label = c("A1", "A2"
), class = "factor"), geometry = structure(list(structure(list(
    list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 
    2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L
    ))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 
    2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 
    0.3, 0.3), .Dim = c(5L, 2L))), list(structure(c(3, 4, 4, 
    3, 3, 2, 3, 3), .Dim = c(4L, 2L)))), class = c("XY", "MULTIPOLYGON", 
"sfg")), structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 
0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 
2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 
0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 
3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L)))), class = c("XY", 
"MULTIPOLYGON", "sfg"))), crs = structure(list(epsg = NA_integer_, 
    proj4string = NA_character_), class = "crs"), n_empty = 0L, precision = 0, bbox = structure(c(xmin = 0, 
ymin = 0, xmax = 4, ymax = 4), class = "bbox"), class = c("sfc_MULTIPOLYGON", 
"sfc"))), row.names = c(NA, -2L), class = "data.frame")

对于我的输出，我遍历id并绘制与每个id对应的多边形列表，所以像这样（没有我的循环）：

ggplot() + 
  geom_sf(data = df[df$id=="A1",])

我想一般地标记每个多边形列表，因此对于每个图，我将具有“片段1”，“片段2” ...等。就像在我的第一张图片中一样，取决于片段数（在下面的示例中，id = A1为3）。  

似乎很基本，但无法弄清楚吗？

Answer 1

出于绘制目的，您需要做的是使用st_cast将MULTIPOLYGON几何图形拆分为多个POLYGON几何图形。默认情况下，这会警告您正在跨几何图形复制属性（此处为id），在这种情况下可以这样做，但根据属性可能会导致错误（请勿复制面积之类的度量！）每行只有一个几何图形，使用geom_sf_label以相同的方式进行绘制变得很容易。

library(tidyverse)
library(sf)
#> Linking to GEOS 3.6.1, GDAL 2.1.3, PROJ 4.9.3
df <- structure(list(id = structure(1:2, .Label = c("A1", "A2"), class = "factor"), geometry = structure(list(structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L))), list(structure(c(3, 4, 4, 3, 3, 2, 3, 3), .Dim = c(4L, 2L)))), class = c("XY", "MULTIPOLYGON", "sfg")), structure(list(list(structure(c(0, 1, 3, 2, 1, 0, 0, 0, 2, 4, 4, 0), .Dim = c(6L, 2L)), structure(c(1, 1, 2, 1, 1, 2, 2, 1), .Dim = c(4L, 2L))), list(structure(c(3, 4, 4, 3, 3, 0, 0, 1, 1, 0), .Dim = c(5L, 2L)), structure(c(3.3, 3.3, 3.8, 3.8, 3.3, 0.3, 0.8, 0.8, 0.3, 0.3), .Dim = c(5L, 2L)))), class = c("XY", "MULTIPOLYGON", "sfg"))), crs = structure(list(epsg = NA_integer_, proj4string = NA_character_), class = "crs"), n_empty = 0L, precision = 0, bbox = structure(c(xmin = 0, ymin = 0, xmax = 4, ymax = 4), class = "bbox"), class = c("sfc_MULTIPOLYGON", "sfc"))), row.names = c(NA, -2L), class = "data.frame")
df %>%
  st_as_sf %>%
  st_cast("POLYGON", group_or_split = TRUE, warn = FALSE) %>%
  ggplot() +
  geom_sf(aes(fill = id)) +
  geom_sf_label(aes(label = id))

在您的示例中，这看起来有点奇怪，因为A2多边形与A1多边形相同，因此颜色和标签被遮盖了。确实绘制了3个A1标签，如果在filter(id == "A1")调用之前添加一行ggplot，则可以看到此标签：

df %>%
  st_as_sf %>%
  st_cast("POLYGON", group_or_split = TRUE, warn = FALSE) %>%
  filter(id == "A1") %>%
  ggplot() +
  geom_sf(aes(fill = id)) +
  geom_sf_label(aes(label = id))

^{由reprex package（v0.2.1）于2019-05-13创建}