How to add labels on top of polygons in leaflet

Question

I am working with the leaflet R package. I have a zoning system made of polygons and I'd like to lay their IDs on top of them. Below is an illustration (with another software) of my objective.

Thanks for your suggestions!

Answer 1

Since there is no reproducible data, I decided to use one of my previous posts related to leaflet. There are two things you want to take away from this post: 1) you need to create a data frame containing center points of target regions, 2) you need to use addLabelOnlyMarkers(). You can achieve the first thing using gCentroid(). I added row names of the polygon data set (UK) as character to centers. This is used for labeling. You need to think what labels you use in your own case. Once this data set is ready, you want to use it in addLabelOnlyMarkers().

library(raster)
library(rgeos)
library(leaflet)

# Get UK polygon data
UK <- getData("GADM", country = "GB", level = 2)

# Find a center point for each region
centers <- data.frame(gCentroid(UK, byid = TRUE))
centers$region <- row.names(UK)

### Create dummy data
set.seed(111)
mydf <- data.frame(place = unique(UK$NAME_2),
                   value = sample.int(n = 1000, size = n_distinct(UK$NAME_2), replace = TRUE))

### Create five colors for fill
mypal <- colorQuantile(palette = "RdYlBu", domain = mydf$value, n = 5, reverse = TRUE)

leaflet() %>% 
addProviderTiles("OpenStreetMap.Mapnik") %>%
setView(lat = 55, lng = -3, zoom = 6) %>%
addPolygons(data = UK,
            stroke = FALSE, smoothFactor = 0.2, fillOpacity = 0.3,
            fillColor = ~mypal(mydf$value),
            popup = paste("Region: ", UK$NAME_2, "<br>",
                          "Value: ", mydf$value, "<br>")) %>%
addLabelOnlyMarkers(data = centers,
                    lng = ~x, lat = ~y, label = ~region,
                    labelOptions = labelOptions(noHide = TRUE, direction = 'top', textOnly = TRUE)) %>%
addLegend(position = "bottomright", pal = mypal, values = mydf$value,
          title = "UK value",
          opacity = 0.3)