Grid with choropleth maps in ggplot2

Question

Using Hadley great ggplot2 and his book (pp. 78-79), I can easily create single choropleth graphics cards using this code:

states.df <- map_data("state") states.df = subset(states.df,group!=8) # get rid of DC states.df$st <- state.abb[match(states.df$region,tolower(state.name))] # attach state abbreviations states.df$value = value[states.df$st] p = qplot(long, lat, data = states.df, group = group, fill = value, geom = "polygon", xlab="", ylab="", main=main) + opts(axis.text.y=theme_blank(), axis.text.x=theme_blank(), axis.ticks = theme_blank()) + scale_fill_continuous (name) p2 = p + geom_path(data=states.df, color = "white", alpha = 0.4, fill = NA) + coord_map(project="polyconic") 

Where the “value” is the state vector of data that I draw. But what if I want to build several cards grouped by one variable (or two)?

Here's an example of a plot made by Andrew Gelman, later adapted by the New York Times , on the state of public health opinion:

I would like to be able to emulate this example: show choropleth plots bound to a grid according to two variables (or even one). Therefore, I do not transmit a vector of values, but a data block organized by "long", with several records for each state.

I know ggplot2 can do this, but I'm not sure how to do it. Thanks!

Answer 1

You can add two columns for the desired groupings and use faces:

 library(ggplot2) library(maps) d1 <- map_data("state") d2 <- unique(d1$group) n <- length(d2) d2 <- data.frame( group=rep(d2,each=6), g1=rep(1:3,each=2,length=6*n), g2=rep(1:2,length=6*n), value=runif(6*n) ) d <- merge(d1, d2, by="group") qplot( long, lat, data = d, group = group, fill = value, geom = "polygon" ) + facet_wrap( ~ g1 + g2 ) 

Answer 2

I just paste this script here in bulk. This is self-sufficient, and I just generate arbitrary categorical variables and a random DV, according to which the colors are colored. There are some things in the code that are not needed; My apologies for this.

 rm(list = ls()) install.packages("ggplot2") library(ggplot2) install.packages("maps") library(maps) install.packages("mapproj") library(mapproj) install.packages("spatstat") library(spatstat) theme_set(theme_bw(base_size = 8)) options(scipen = 20) MyPalette <- colorRampPalette(c(hsv(0, 1, 1), hsv(7/12, 1, 1))) ### Map ### StateMapData <- map_data("state") head(StateMapData) ### Some Invented Data ### IndependentVariable1 <- c("Low Income", "Mid Income", "High Income") IndependentVariable2 <- c("18-29", "30-44", "45-64", "65+") # Here is one way to "stack" lots of copies of the shapefile dataframe on top of each other: # This needs to be done, because (as far as I know) ggplot2 needs to have the state names and polygon coordinates # for each level of the faceting variables. TallData <- expand.grid(1:nrow(StateMapData), IndependentVariable1, IndependentVariable2) TallData <- data.frame(StateMapData[TallData[, 1], ], TallData) colnames(TallData)[8:9] <- c("IndependentVariable1", "IndependentVariable2") # Some random dependent variable we want to plot in color: TallData$State_IV1_IV2 <- paste(TallData$region, TallData$IndependentVariable1, TallData$IndependentVariable2) RandomVariable <- runif(length(unique(TallData$State_IV1_IV2))) TallData$DependentVariable <- by(RandomVariable, unique(TallData$State_IV1_IV2), mean)[TallData$State_IV1_IV2] ### Plot ### MapPlot <- ggplot(TallData, aes(x = long, y = lat, group = group, fill = DependentVariable)) MapPlot <- MapPlot + geom_polygon() MapPlot <- MapPlot + coord_map(project="albers", at0 = 45.5, lat1 = 29.5) # Changes the projection to something other than Mercator. MapPlot <- MapPlot + scale_x_continuous(breaks = NA, expand.grid = c(0, 0)) + scale_y_continuous(breaks = NA) + opts( panel.grid.major = theme_blank(), panel.grid.minor = theme_blank(), panel.background = theme_blank(), panel.border = theme_blank(), expand.grid = c(0, 0), axis.ticks = theme_blank(), legend.position = "none", legend.box = "horizontal", title = "Here is my title", legend.key.size = unit(2/3, "lines")) MapPlot <- MapPlot + xlab(NULL) + ylab(NULL) MapPlot <- MapPlot + geom_path(fill = "transparent", colour = "BLACK", alpha = I(2/3), lwd = I(1/10)) MapPlot <- MapPlot + scale_fill_gradientn("Some/nRandom/nVariable", legend = FALSE, colours = MyPalette(100)) # This does the "faceting": MapPlot <- MapPlot + facet_grid(IndependentVariable2 ~ IndependentVariable1) # print(MapPlot) ggsave(plot = MapPlot, "YOUR DIRECTORY HERE.png", h = 8.5, w = 11)