label for overlapping polygons in R leaflet - r

Question

I need to label several overlapping polygons, but only the label of the biggest one is shown. However when I tested with some simulated data the labels were shown correctly. I compared the data in two cases carefully but cannot find the difference caused the problem.
Here is a minimal example of simulated overlapping polygons:
library(leaflet)
library(sp)
poly_a <- data.frame(lng = c(0, 0.5, 2, 3),
lat = c(0, 4, 4, 0))
poly_b <- data.frame(lng = c(1, 1.5, 1.8),
lat = c(2, 3, 2))
pgons = list(
Polygons(list(Polygon(poly_a)), ID="1"),
Polygons(list(Polygon(poly_b)), ID="2")
)
poly_dat <- data.frame(name = as.factor(c("a", "b")))
rownames(poly_dat) <- c("1", "2")
spgons = SpatialPolygons(pgons)
spgonsdf = SpatialPolygonsDataFrame(spgons, poly_dat, TRUE)
leaflet() %>% addPolygons(data = spgonsdf, label = ~name
# ,
# highlightOptions = highlightOptions(
# color = "red", weight = 2,bringToFront = TRUE)
)
It's working properly:
However it didn't work with my data.
https://github.com/rstudio/leaflet/files/1430888/Gabs.zip
You can drag the zip into this site and use the i button to see it's correctly labeled
library(rgdal)
# download Gabs.zip and extract files to Gabs folder
hr_shape_gabs <- readOGR(dsn = 'Gabs', layer = 'Gabs - OU anisotropic')
hr_shape_gabs_pro <- spTransform(hr_shape_gabs,
CRS("+proj=longlat +datum=WGS84 +no_defs"))
leaflet(hr_shape_gabs_pro) %>%
addTiles() %>%
addPolygons(weight = 1, label = ~name)
Only the biggest polygon label is shown:
The data in both case are SpatialPolygonsDataFrame, the data slot have proper polygon names.

Answer 1

Change the order of polygons in hr_shape_gabs: polygon in position 3 should be the smaller one.
library(leaflet)
library(sp)
library(rgdal)
hr_shape_gabs <- readOGR(dsn = 'Gabs - OU anisotropic.shp',
layer = 'Gabs - OU anisotropic')
# Change the position of the smaller and wider polygons
# Position 1 = wider polygon, position 3 = smaller polygon
pol3 <- hr_shape_gabs#polygons[[3]]
hr_shape_gabs#polygons[[3]] <- hr_shape_gabs#polygons[[1]]
hr_shape_gabs#polygons[[1]] <- pol3
hr_shape_gabs$name <- rev(hr_shape_gabs$name)
hr_shape_gabs_pro <- spTransform(hr_shape_gabs,
CRS("+proj=longlat +datum=WGS84 +no_defs"))
leaflet() %>%
addTiles() %>%
addPolygons(data= hr_shape_gabs_pro, weight = 1, label = ~name)

Answer 2

Here's a scalable solution in sf for many layers, based on this answer.
The idea is to order the polygons by decreasing size, such that the smallest polygons plot last.
library(sf)
library(dplyr)
# calculate area of spatial polygons sf object
poly_df$area <- st_area(poly_df)
poly_df <- arrange(poly_df, -area)
# view with labels in leaflet to see that small polygons plot on top
leaflet(poly_df) %>% addTiles() %>% addPolygons(label = ~id)
Apologies for the lack of reproducibility. This is more of a concept answer.