How to plot a specific column from a SpatialPolygonDataframe with leafletR? - r

Question

I would like to create a choropleth map using leafletR::leaflet.
My data comes in a SpatialPolygonsDataFrame, and I would like to choose a specific column to be plotted.
With sp::spplot, this is easy-peasy, since the argument zcol allows me to specify the layer/column to be plotted:
library("maptools");library("sp");library("leafletR")
SP <- readShapePoly(system.file("shapes/sids.shp",
package="maptools")[1],
proj4string=CRS("+proj=longlat +datum=WGS84
+no_defs +ellps=WGS84
+towgs84=0,0,0"))
spplot(SP, zcol="BIR79")
However, with leafletR, I don't know how to specify the layer, and it just plots the plain map borders:
SP4leaflet <- toGeoJSON(data=SP, dest=tempdir(), name="BIR79")
SPleaflet <- leaflet(data=SP4leaflet, dest=tempdir(),
title="Trying to plot BIR79",
base.map="osm", popup="*")
SPleaflet
Any ideas on how to select the desired layer/column to be plotted with leafletR?

Answer 1

For the sake of completeness, I would like to mention that this is now easily possible using library(mapview).
library("maptools")
library("sp")
library("mapview")
SP <- readShapePoly(system.file("shapes/sids.shp",
package="maptools")[1],
proj4string=CRS("+proj=longlat +datum=WGS84
+no_defs +ellps=WGS84
+towgs84=0,0,0"))
spplot(SP, zcol="BIR79")
mapview(SP, zcol="BIR79")
See also http://edzer.github.io/sp/#interactive-maps-leaflet-mapview for a similar example and http://environmentalinformatics-marburg.github.io/web-presentations/20150723_mapView.html for more functionality of mapview.

Answer 2

Your code lacks a style definition. For a graduated style you have to create a style object using styleGrad. The prop parameter in styleGrad specifies the property you want to visualize.
library("maptools")
library("sp")
library("leafletR")
SP <- readShapePoly(system.file("shapes/sids.shp",
package="maptools")[1],
proj4string=CRS("+proj=longlat +datum=WGS84
+no_defs +ellps=WGS84
+towgs84=0,0,0"))
SP4leaflet <- toGeoJSON(data=SP, dest=tempdir(), name="BIR79")
SPleaflet <- leaflet(data=SP4leaflet, dest=tempdir(),
title="Trying to plot BIR79",
base.map="osm", popup="*")
## missing style definition
brks <- seq(0, max(SP$BIR79), by=5000)
clrs <- colorRampPalette(c("blue","yellow", "red"))(7)
stl <- styleGrad(prop="BIR79", breaks=brks, style.val=clrs,
out=1, leg="BIR79")
## end style definition
SPleaflet <- leaflet(data=SP4leaflet, dest=tempdir(),
title="Trying to plot BIR79", base.map="osm",
style=stl, popup="*")
SPleaflet

Answer 3

Maybe something like this works. Note that I used leaflet not leafletR. There is a good intro about the package here: here
library(leaflet)
library(maptools)
library(sp)
library(classInt)
library(classInt)
SP <- readShapePoly(system.file("shapes/sids.shp",
package="maptools")[1],
proj4string=CRS("+proj=longlat +datum=WGS84
+no_defs +ellps=WGS84
+towgs84=0,0,0"))
spplot(SP, zcol="BIR79")
## 6 classes with fixed given breaks
nclass <- classIntervals(SP$BIR79, n=6)
## Color for each class
colcode = findColours(nclass, c("red", "blue"))
leaflet(SP) %>% addTiles() %>% addPolygons(data = SP, color = colcode)