Leaflet

library(simplevis)
library(stars)
set.seed(123456789)

Overview

{leaflet} is a library that provides beautiful html widget maps. This allow use of basemaps, and therefore the ability to zoom in on features with the context that these provide. {simplevis} provides wrapper functions to map sf and stars objects. Arguments in these have been aligned to {ggplot2} terminology to make things simple for the user.

For the sf functions:

Data must be an sf object
Data must be of POINT/MULTIPOINT, LINESTRING/MULTILINESTRING, or POLYGON/MULTIPOLYGON geometry type
Data must have a coordinate reference system (CRS) defined
No x_var and y_var variables are required

For the stars functions:

Data must be a stars object
Data must have a coordinate reference system (CRS) defined
No x_var and y_var variables are required

There is no facetting functionality provided for leaflet.

Colouring

Colouring is the same as that for graphs.

leaf_sf_col(example_point, 
            col_var = trend_category)

leaf_sf_col(example_polygon,  
            col_var = density)

leaf_stars_col(example_stars, 
               col_var = nitrate, 
               col_method = "bin", 
               col_breaks_n = 4, 
               col_na_rm = TRUE)

leaf_stars_col(example_stars, 
               col_var = nitrate, 
               col_method = "bin", 
               col_cuts = c(0, 500, 1000, 2000, Inf), 
               col_na_rm = TRUE)

leaf_sf_col(example_polygon, 
            col_var = density, 
            col_method = "quantile", 
            col_breaks_n = 4)

leaf_sf_col(example_polygon, 
            col_var = density, 
            col_method = "quantile", 
            col_cuts = c(0, 0.25, 0.5, 0.75, 0.95, 1))

leaf_stars_col(example_stars, 
               col_var = nitrate, 
               col_method = "bin", 
               col_breaks_n = 7, 
               col_na_rm = TRUE)

sf popups and hover text

The clickable popup will default to a leafpop::popupTable of all variables, but popups can be adjusted to a subset of column using the popup_vars_vctr argument.

leaf_sf_col(example_point, 
            col_var = trend_category, 
            popup_vars_vctr = c("site_id", "median"))

The hover label will default to the colour variable, but can be adjusted using the label_var variable.

leaf_sf_col(example_point, 
            col_var = trend_category, 
            label_var = site_id)

Basemaps

Users have a basemap argument that defaults to “light”, but there are other options.

leaf_sf(example_point, 
        basemap = "dark")

Legend labels

Legend labels are not as flexible as in the ggplot2 wrapper functions. You can adjust by using the col_labels argument, and providing a manual vector of breaks.

leaf_sf_col(example_point, 
            col_var = trend_category, 
            col_labels = c("Better", "Same", "Worse"))

Adding additional layers

As a leaflet object is produced, you can add additional layers with leaflet - although this may effect popups and labels. Note all objects provided to leaflet functions must be transformed to a CRS of 4326.

leaf_sf_col(example_point, 
            col_var = trend_category) %>% 
  leaflet::addPolygons(data = sf::st_transform(example_borders, 4326),
                       color = "#35B779", 
                       weight = 3, 
                       fillOpacity = 0, 
                       opacity = 1)

leaflet basemap stack for shiny

A leaflet basemap stack is available for use in shiny apps. It defaults to the top layer being “light”. You can set the bounds by adding a vector or bbox of bounds.

leaf_basemap(bounds = c(166.70047,-34.45676, 178.52966,-47.06345))

You can also specify the basemap top layer.

bb <- rnaturalearth::ne_countries(scale = "small", 
                                  country = "Indonesia", 
                                  returnclass = "sf") %>% 
   sf::st_bbox() 
 
leaf_basemap(bounds = bb, basemap = "satellite")

Layer id’s for use in shiny

You can add a layer_id variable to leaf_sf* functions for use in shiny, which enables you to return a value when the value is clicked on.

This enables you to program shiny to do specific things when a feature is clicked upon.

For example, if the server.R code downloaded from shiny_demo() is updated as follows, a modal dialog can be created with a graph that compares a feature value with the distribution of all values.

In server.R, adjust your leaf_sf* function by adding a layer_id variable and then turning the default popup to FALSE.

  leaf_sf_col(
    leaf_data(),
    col_var = trend_category,
    size_point = size_reactive,
    col_title = title, 
    layer_id_var = site_id,
    popup = FALSE
  )

Then in server.R, add ObserveEvent code to observe when a feature is clicked on, and then add some code to put something something in a modal dialog box.

  observeEvent(input$leaf_marker_click, {
    
    site_median <- leaf_data() %>% 
      filter(site_id == input$leaf_marker_click$id) %>% 
      pull(median)
    
    showModal(
      modalDialog(
        renderPlot(
          gg_histogram(example_point, median, 
                       title = paste0(input$leaf_marker_click$id, " compared to all sites"), 
                       theme = gg_theme(font = "Helvetica", size_title = 16,  size_body = 15, gridlines_h = TRUE)) +
            ggplot2::geom_vline(ggplot2::aes(xintercept = site_median), col = "red")
        ),
        easyClose = TRUE
      )
    )
  })

Note that the input$leaf_marker_click$id returns the value of the variable assigned to the layer_id. The click event returns input$leaf_marker_click in this example because the map_id defaults to “leaf” and this relates to a circlemarker. Refer here for more information.

David Hodge

2023-05-09