Add leaflet legend for polygon outlines

Question

Is there a mechanism for adding legends for for polygon outlines? I can map hollow polygon outlines by altering the color field and specifying fillOpacity=0 in my addPolygons() call, however I can achieve the same functionality when I addLegend() to the map? Example of hollow polygons and filled legend below:

library(sf)
library(leaflet)
library(tidyverse)

demo(nc, ask = FALSE, echo = FALSE)

nc_pal <- colorFactor(palette = "inferno", domain = nc$Name)

nc <- st_transform(nc, 4326)

leaflet(nc) %>%
  addProviderTiles(provider = c("Esri.WorldTopoMap"),
                   group = "Esri.WorldTopoMap") %>%
  addPolygons(
    data = nc,
    group = "nc",
    color = ~ nc_pal(NAME),
    fillOpacity = 0
  ) %>%
  addLegend(
    data = nc,
    group = "nc",
    colors = ~ nc_pal(NAME),
    labels = ~ NAME
  )

Same layout but having a "hollow" colour patch to indicate that legend is only for the outlines. — BEVAN, Aug 14 '23 at 17:41

score 1 · Accepted Answer · answered Aug 15 '23 at 09:41

You could use the function from this question which creates a custom legend by making the inner filled squares white. Here is some reproducible code:

library(sf)
library(leaflet)
library(tidyverse)

leaflet(nc) %>%
  addProviderTiles(provider = c("Esri.WorldTopoMap"),
                   group = "Esri.WorldTopoMap") %>%
  addPolygons(
    data = nc,
    group = "nc",
    color = ~ nc_pal(NAME),
    fillOpacity = 0
  ) %>%
  addLegendCustom(colors = rep("white", n = length(nc)), 
                  labels = nc$NAME, 
                  sizes = 20, 
                  shapes = "square", 
                  borders = nc_pal(nc$NAME))

The function from the question used:

addLegendCustom <- function(map, colors, labels, sizes, shapes, borders, opacity = 0.5){
  
  make_shapes <- function(colors, sizes, borders, shapes) {
    shapes <- gsub("circle", "50%", shapes)
    shapes <- gsub("square", "0%", shapes)
    paste0(colors, "; width:", sizes, "px; height:", sizes, "px; border:3px solid ", borders, "; border-radius:", shapes)
  }
  make_labels <- function(sizes, labels) {
    paste0("<div style='display: inline-block;height: ", 
           sizes, "px;margin-top: 4px;line-height: ", 
           sizes, "px;'>", labels, "</div>")
  }
  
  legend_colors <- make_shapes(colors, sizes, borders, shapes)
  # I added my modification here, see below
  legend_labels <- make_labels(sizes, labels)
  
  return(addLegend(map, colors = legend_colors, labels = legend_labels, opacity = opacity))
}

^{Created on 2023-08-15 with reprex v2.0.2}

1

That works great, thank you. – BEVAN Aug 15 '23 at 22:55

Add leaflet legend for polygon outlines

1 Answers1