Plot a cartogram for the USA using crime data by joining a plain data frame to a SpatialPolygonsDataFrame

cartogram-us-states.R

require(maps)
require(sp)
require(maptools)
require(tmap)
require(cartogram)

# get a SpatialPolygonsDataFrame of US states
usa <- map("state", fill = TRUE)
IDs <- sapply(strsplit(usa$names, ":"), function(x) x[1])
usa <- map2SpatialPolygons(usa, IDs=IDs, proj4string=CRS("+proj=longlat +datum=WGS84"))
usa <- SpatialPolygonsDataFrame(usa, 
                                data = data.frame(unique(IDs), 
                                                  row.names = unique(IDs)) )

# get a data frame of data with state names. Data here are crime rates,
# built-in dataset
crimes <- data.frame(state = tolower(rownames(USArrests)), USArrests)

# join data frame to the SpatialPolygonsDataFrame
# from https://stackoverflow.com/a/3652472/1036500
usa@data = data.frame(usa@data, crimes[match(usa@data[,'unique.IDs.'], crimes[,'state' ]),])

# make cartogram
crimes_ctgm <- cartogram(usa, "Murder", itermax=5)

# plot it
tm_shape(crimes_ctgm) + 
  tm_fill("Murder", style="jenks") +
  tm_borders() + 
  tm_layout(frame=F)