How to display a custom color for NA values in an R Plotly choropleth map?

Question

I would like to change the fill color of NA values in a choropleth r plotly interactive map from white (or transparent?) to light gray. This is for an R Shiny application, so I would like to retain the map interactivity features of plotly. I am using custom shapes using the sf package. Does anyone have a solution for this? Thank you.

I have a reproducible example here:

library(sf)
library(dplyr)
library(plotly)

nc <- sf::st_read(system.file("shape/nc.shp", package="sf"), quiet = TRUE) %>% 
        mutate(NA.AREA = ifelse(AREA >  0.15, NA, AREA))


plot_ly(nc,
        split = ~NAME,
        color = ~NA.AREA,
        colors =  'Blues',
        alpha = 1,
        stroke = I("#000000"), 
        span = I(1),
        hovertext = ~NAME, 
        hoverinfo = "text",
        showlegend = FALSE)

Example r plotly choropleth map with NA values:

I see there is a way to do this in ggplot2, but I am not seeing any documentation on how to do this in plotly for R.

Answer 1

We can filter the data for NA and non-NA values, and use add_sf to plot them separately with desired colors:

library(sf)
library(dplyr)
library(plotly)

nc <- sf::st_read(system.file("shape/nc.shp", package="sf"), quiet = TRUE) %>%
         mutate(NA.AREA = ifelse(AREA >  0.15, NA, AREA))

plot_ly()  %>% 
  add_sf(data = nc %>%  filter(!is.na(NA.AREA)),
         split = ~NAME,
         color = ~NA.AREA,
         colors =  'Blues',
         alpha = 1,
         stroke = I("#000000"), 
         span = I(1),
         hovertext = ~NAME, 
         hoverinfo = "text",
         showlegend = FALSE) %>% 
  add_sf(data = nc %>%  filter(is.na(NA.AREA)),
            split = ~NAME,
            color = I("grey"),
            alpha = 1,
            stroke = I("#000000"), 
            span = I(1),
            hovertext = ~NAME, 
            hoverinfo = "text",
            showlegend = FALSE)

^{Created on 2024-07-19 with reprex v2.0.2}