Legends

The Leaflet package includes convenience functions for creating color legends. In this section, we will build on the example from the Colors page.

library(rgdal)

# From http://data.okfn.org/data/datasets/geo-boundaries-world-110m
countries <- readOGR("json/countries.geojson", "OGRGeoJSON")

## OGR data source with driver: GeoJSON 
## Source: "json/countries.geojson", layer: "OGRGeoJSON"
## with 177 features
## It has 2 fields

map <- leaflet(countries) %>% addTiles()

Use the addLegend function to add a legend. The easiest way to use addLegend is to provide pal (a palette function, as generated from colorNumeric et al.) and values, and let it calculate the colors and labels for you.

In most cases you will simply be separating the function and argument you passed into addPolygons(color=...), as in this example:

pal <- colorNumeric(
  palette = "YlGnBu",
  domain = countries$gdp_md_est
)
map %>%
  addPolygons(stroke = FALSE, smoothFactor = 0.2, fillOpacity = 1,
    color = ~pal(gdp_md_est)
  ) %>%
  addLegend("bottomright", pal = pal, values = ~gdp_md_est,
    title = "Est. GDP (2010)",
    labFormat = labelFormat(prefix = "$"),
    opacity = 1
  )

The addLegend() function is aware of the different types of palette functions, and will create an appropriate default rendering for each type. For example, contrast the legend created for the colorNumeric-based palette above with the colorQuantile-based palette below. The latter shows probability ranges, with a value range tooltip.

qpal <- colorQuantile("RdYlBu", countries$gdp_md_est, n = 5)
map %>%
  addPolygons(stroke = FALSE, smoothFactor = 0.2, fillOpacity = 1,
    color = ~qpal(gdp_md_est)
  ) %>%
  addLegend(pal = qpal, values = ~gdp_md_est, opacity = 1)

addLegend() has several other parameters that allows you to customize the legend in various ways. Rather than using pal and values, you can explicitly pass in colors and labels. You can change the title and color opacity.

You can also conveniently customize the label appearance by passing labFormat=labelFormat(). labelFormat() has parameters that customize the separator between ranges, the number of digits to render, and prefix/suffix for each label. If your label formatting needs extend beyond what labelFormat() can provide, you can also use a custom function as the labFormat argument; see the Details section in ?addLegend for a description.