Spatial Plot in R : how to plot the polygon and color as per the data to be visualized

Question

I have been trying to draw the county based Choropleth map in R for visualizing my dataset for the State of Arizona.

For plotting the thematic map using the polygon bases data for the county from arizona.edu (Spatial Library) and data is from az.gov

It have the following for plotting the COUNTY polygon-

library(maptools)
library(rgdal)
library(ggplot2)
library(plyr)
county <- readShapePoly(file.choose())
county@data$id <- rownames(county@data)
county.points <- fortify(county, coords="id")
county.df <- join(county.points, county@data, by="id")
ggplot(county.df) + aes(long,lat,group=group, fill="id") + 
geom_polygon() +
geom_path(color="white") +
coord_equal() +
scale_fill_brewer("County Arizona")

This code is not giving me any error and also no output.

My Source of Shape file here

Data Source here

Answer 1

I can't speak to why your code is not generating output - there are too many possible reasons - but is this what you are trying to achieve?

Code

library(rgdal)
library(ggplot2)
library(plyr)
library(RColorBrewer)
setwd("< directory with all your files >")

map       <- readOGR(dsn=".",layer="ALRIS_tigcounty")
marriages <- read.csv("marriages.2012.csv",header=T,skip=3)
marriages <- marriages[2:16,]
marriages$County <- tolower(gsub(" ","",marriages$County))
marriages$Total  <- as.numeric(as.character(marriages$Total))

data <- data.frame(id=rownames(map@data), NAME=map@data$NAME, stringsAsFactors=F)
data <- merge(data,marriages,by.x="NAME",by.y="County",all.x=T)
map.df <- fortify(map)
map.df <- join(map.df,data, by="id")

ggplot(map.df, aes(x=long, y=lat, group=group))+
  geom_polygon(aes(fill=Total))+
  geom_path(colour="grey50")+
  scale_fill_gradientn("2012 Marriages",
                       colours=rev(brewer.pal(8,"Spectral")), 
                       trans="log", 
                       breaks=c(100,300,1000,3000,10000))+
  theme(axis.text=element_blank(), 
        axis.ticks=element_blank(), 
        axis.title=element_blank())+
  coord_fixed()

Explanation

To generate a choropleth map, ultimately we need to associate polygons with your datum of interest (total marriages by county). This is a three step process: first we associate polygon ID with county name:

data <- data.frame(id=rownames(map@data), NAME=map@data$NAME, stringsAsFactors=F)

Then we associate county name with total marriages:

data <- merge(data,marriages,by.x="NAME",by.y="County",all.x=T)

Then we associate the result with the polygon coordinate data:

map.df <- join(map.df,data, by="id")

Your specific case has a lot of potential traps:

1. The link you provided was to a pdf - utterly useless. But poking around a bit revealed an Excel file with the same data. Even this file needs cleaning: the data has "," separators, which need to be turned off, and some of the cells have footnotes, which have to be removed. Finally, we have to save as a csv file.

2. Since we are matching on county name, the names have to match! In the shapefile attributes table, the county names are all lower case, and spaces have been removed (e.g., "Santa Cruz" is "santacruz". So we need to lowercase the county names and remove spaces:

   marriages$County <- tolower(gsub(" ","",marriages$County))

3. The totals column comes in as a factor, which has to be converted to numeric:

   marriages$Total <- as.numeric(as.character(marriages$Total))

4. Your actual data is highly skewed: maricopa county had 23,600 marriages, greenlee had 50. So using a linear color scale is not very informative. Consequently, we use a logarithmic scale:

   scale_fill_gradientn("2012 Marriages", colours=rev(brewer.pal(8,"Spectral")), trans="log", breaks=c(100,300,1000,3000,10000))+