How to render a leaflet choropleth map in shiny? - r

Question

I have successfully created an interactive choropleth map using Leaflet in R that projects a single variable across a set of polygons.
library(RSocrata)
library(rgdal)
library(leaflet)
library(sp)
library(dplyr)
#library(mapview)
area_bound <- rgdal::readOGR("https://data.cityofchicago.org/resource/igwz-8jzy.geojson")
area_bound$area_num_1 <- as.numeric(area_bound$area_numbe)
health <- read.socrata("https://data.cityofchicago.org/resource/iqnk-2tcu.json")
data_num <- as.data.frame(apply(health[3:29], 2, as.numeric))
data_num <- bind_cols(health[1:2], data_num)
health_area <- sp::merge(area_bound, data_num, by.x = "area_numbe", by.y = "community_area")
pal <- colorNumeric("viridis", NULL)
leaflet(health_area) %>%
addTiles() %>%
addPolygons(stroke = FALSE, smoothFactor = 0.3, fillOpacity = 1,
fillColor = ~pal(as.numeric(firearm_related)),
label = ~paste0(community, ": ", formatC(firearm_related, big.mark = ",")))
The health data set has multiple variables and I would like to create a shiny app that allows users to choose a different variable to produce a choropleth map. Using the code provided by Kyle Walker as a model for my server, I came up with the code below that allows users to choose from a list of two variables. Unfortunately I am having problems running it, getting a Warning: Error in min: invalid 'type' (list) of argument error. Any help in resolving this would be appreciated. I have also looked at the RStudio, Using Leaflet With Shiny tutorial, but the examples provided are not choropleth maps.
Here is my non-working code:
## app.R ##
library(shiny) # for shiny apps
library(leaflet) # renderLeaflet function
library(RSocrata)
library(rgdal)
library(sp)
library(dplyr)
area_bound <- rgdal::readOGR("https://data.cityofchicago.org/resource/igwz-8jzy.geojson")
area_bound$area_num_1 <- as.numeric(area_bound$area_numbe)
health <- read.socrata("https://data.cityofchicago.org/resource/iqnk-2tcu.json")
data_num <- as.data.frame(apply(health[3:29], 2, as.numeric))
data_num <- bind_cols(health[1:2], data_num)
health_area <- sp::merge(area_bound, data_num, by.x = "area_numbe", by.y = "community_area")
groups <- c("Breast Cancer" = "breast_cancer_in_females", "Firearm" = "firearm_related")
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
radioButtons(
inputId = "group",
label = "Select a group to map",
choices = groups
)
),
mainPanel(
leafletOutput("map", height = "600")
)
)
)
server = function(input, output) {
group_to_map <- reactive({
input$group
})
output$map <- renderLeaflet({
leaflet(options = leafletOptions(zoomControl = FALSE)) %>%
addProviderTiles(providers$Stamen.TonerLite) %>%
setView(lng = -87.623177,
lat = 41.881832,
zoom = 8.5)
})
observeEvent(input$group, {
pal <- colorNumeric("viridis", group_to_map)
leafletProxy("map") %>%
clearShapes() %>%
clearControls() %>%
addPolygons(data = group_to_map,
color = ~pal(),
weight = 0.5,
fillOpacity = 0.5,
smoothFactor = 0.2) %>%
addLegend(
position = "bottomright",
pal = pal,
values = group_to_map,
title = "% of population"
)
})
}
shinyApp(ui, server)

Answer 1

There are several issues with your shiny code. First, to refer to values from a reactive you have to call it like a function, i.e. you have to do group_to_map(). Next, group_to_map() is just a character. To use the data column whose name is stored in group_to_map() you have to do health_area[[group_to_map()]]. I also fixed the issue with your palette functions. Finally, note that I switched to sf for reading the geo data as I'm more familiar with sf objects:
## app.R ##
library(shiny) # for shiny apps
library(leaflet) # renderLeaflet function
library(RSocrata)
library(dplyr)
area_bound <- sf::st_read("https://data.cityofchicago.org/resource/igwz-8jzy.geojson")
health <- read.socrata("https://data.cityofchicago.org/resource/iqnk-2tcu.json")
health[3:29] <- lapply(health[3:29], as.numeric)
#> Warning in lapply(health[3:29], as.numeric): NAs introduced by coercion
health_area <- left_join(area_bound, health, by = c("area_num_1" = "community_area"))
groups <- c("Breast Cancer" = "breast_cancer_in_females", "Firearm" = "firearm_related")
ui <- fluidPage(
sidebarLayout(
sidebarPanel(
radioButtons(
inputId = "group",
label = "Select a group to map",
choices = groups
)
),
mainPanel(
leafletOutput("map", height = "600")
)
)
)
server = function(input, output) {
group_to_map <- reactive({
input$group
})
output$map <- renderLeaflet({
leaflet(options = leafletOptions(zoomControl = FALSE)) %>%
addProviderTiles(providers$Stamen.TonerLite) %>%
setView(lng = -87.623177,
lat = 41.881832,
zoom = 8.5)
})
observeEvent(input$group, {
pal <- colorNumeric("viridis", range(health_area[[group_to_map()]]))
leafletProxy("map") %>%
clearShapes() %>%
clearControls() %>%
addPolygons(data = health_area,
color = ~pal(health_area[[group_to_map()]]),
weight = 0.5,
fillOpacity = 0.5,
smoothFactor = 0.2) %>%
addLegend(
position = "bottomright",
pal = pal,
values = health_area[[group_to_map()]],
title = "% of population"
)
})
}
shinyApp(ui, server)
#>
#> Listening on http://127.0.0.1:5938