Great Maps with ggplot2
The above map (and this one) was produced using R and ggplot2 and serve to demonstrate just how sophisticated R visualisations can be. We are used to seeing similar maps produced with conventional GIS platforms or software such as Processing but I hadn’t yet seen one from the R community (feel free to suggest some in the comments). The map contains three layers: buildings, water and the journey segments. The most challenging aspect was to change the standard line ends in geom_segment from “butt” to “round” in order that the lines appeared continuous and not with “cracks” in, see below.
I am grateful to Hadley and the rest of the ggplot2 Google Group for the solution. You can see it here. From this point I layered the plots using the geom_polygon() command for the buildings and water bodies and my new function geom_segment2() for the journey segments- these were simply the start and end latitudes and longitudes for each node in the road network and the number of times a cyclist passed between them. I have included the code below
#Code supplied by james cheshire Feb 2012
#load packages and enter development mode
library('devtools')
dev_mode()
library(ggplot2)
library(proto)
#if your map data is a shapefile use maptools
library(maptools)
gpclibPermit()
#create GeomSegment2 function
GeomSegment2 <- proto(ggplot2:::GeomSegment, {
objname <- "geom_segment2"
draw <- function(., data, scales, coordinates, arrow=NULL, ...) {
if (is.linear(coordinates)) {
return(with(coord_transform(coordinates, data, scales),
segmentsGrob(x, y, xend, yend, default.units="native",
gp = gpar(col=alpha(colour, alpha), lwd=size * .pt,
lty=linetype, lineend = "round"),
arrow = arrow)
))
}
}})
geom_segment2 <- function(mapping = NULL, data = NULL, stat =
"identity", position = "identity", arrow = NULL, ...) {
GeomSegment2$new(mapping = mapping, data = data, stat = stat,
position = position, arrow = arrow, ...)
}
#load data stlat/stlong are the start points elat/elong are the end points of the lines
lon<- read.csv("bikes_london.csv", header=F, sep=";")
names(lon)<-c("stlat", "stlon", "elat", "elong", "count")
#load spatial data. You need to fortify if loaded as a shapefile
water<- fortify(readShapePoly("waterfeatures.shp"))
built<- fortify(readShapePoly("buildings.shp"))
#This step removes the axes labels etc when called in the plot.
xquiet<- scale_x_continuous("", breaks=NA)
yquiet<-scale_y_continuous("", breaks=NA)
quiet<-list(xquiet, yquiet)
#create base plot
plon1<- ggplot(lon, aes(x=stlon, y=stlat))
#ready the plot layers
pbuilt<-c(geom_polygon(data=built, aes(x=long, y=lat, group=group), colour= "#4B4B4B", fill="#4F4F4F", lwd=0.2))
pwater<-c(geom_polygon(data=water, aes(x=long, y=lat, group=group), colour= "#708090", fill="#708090"))
#create plot
plon2<- plon1 +pbuilt+ pwater+ geom_segment2(aes(xend=elong, yend=elat, size= count, colour=count))+scale_size(range=c(0.06, 1.8))+scale_colour_gradient(low="#FFFFFF", high="#FFFF33", space="rgb")+coord_equal(ratio=1/cos(lon$elat[1]*pi/180))+quiet+ opts(panel.background=theme_rect(fill="#404040"))
plon2