mapplot: Trellis displays on Maps a.k.a. Choropleth maps

Description

Produces Trellis displays of numeric (and eventually categorical) data on a map. This is largely meant as a demonstration, and users looking for serious map drawing capabilities should look elsewhere (see below).

Usage

mapplot(x, data, …)
# S3 method for formula
mapplot(x, data, map, outer = TRUE,
        prepanel = prepanel.mapplot,
        panel = panel.mapplot,
        aspect = "iso",
        legend = NULL,
        breaks, cuts = 30,
        colramp = colorRampPalette(brewer.pal(n = 11, name = "Spectral")),
        colorkey = TRUE,
        …)

prepanel.mapplot(x, y, map, …)
panel.mapplot(x, y, map, breaks, colramp, exact = FALSE, lwd = 0.5, …)

Arguments

x, y

For mapplot, an object on which method dispatch is carried out. For the formula method, a formula of the form y ~ x, with additional conditioning variables as desired. The extended form of conditioning using y ~ x1 + x2 etc. is also allowed. The formula might be interpreted as in a dot plot, except that y is taken to be the names of geographical units in map.

Suitable subsets (packets) of x and y are passed to the prepanel and panel functions.

data

A data source where names in the formula are evaluated

map

An object of class "map" (package maps), containing boundary information. The names of the geographical units must match the y variable in the formula.

outer

logical; how variables separated by + in the formula are interpreted. It is not advisable to change the default.

prepanel, panel

the prepanel and panel functions

aspect

aspect ratio

breaks, cuts, colramp

controls conversion of numeric x values to a false color. colramp should be a function that produces colors (such as cm.colors). If it is NULL, colors are taken from trellis.par.get("regions").

exact

the default exact = FALSE allows the given y values to match sub-regions of map, i.e. region names with a qualifier following ":", like "michigan:north", "michigan:south". These will both match a y value of "Michigan".

legend, colorkey

controls legends; usually just a color key giving the association between numeric values of x and color.

lwd

line width

…

Further arguments passed on to the underlying engine. See xyplot for details.

Value

An object of class "trellis".

References

http://en.wikipedia.org/wiki/Choropleth_map

Examples

Run this code

# NOT RUN {
library(maps)
library(mapproj)

## Note: Alaska, Hawaii and others are not included in county map;
## this generates warnings with both USCancerRates and ancestry.

data(USCancerRates)

suppressWarnings(print(
mapplot(rownames(USCancerRates) ~ log(rate.male) + log(rate.female),
        data = USCancerRates,
        map = map("county", plot = FALSE, fill = TRUE,
                  projection = "mercator"))
))

suppressWarnings(print(
mapplot(rownames(USCancerRates) ~ log(rate.male) + log(rate.female),
        data = USCancerRates,
        map = map("county", plot = FALSE, fill = TRUE,
                  projection = "tetra"),
        scales = list(draw = FALSE))
))

data(ancestry)

county.map <- 
    map('county', plot = FALSE, fill = TRUE, 
        projection = "azequalarea")

## set a sequential color palette as current theme, and use it
opar <- trellis.par.get()
trellis.par.set(custom.theme(region = brewer.pal(9, "Purples"),
                             alpha.line = 0.5))
suppressWarnings(print(
mapplot(county ~ log10(population), ancestry, map = county.map,
    colramp = NULL)
))
trellis.par.set(opar)

# }
# NOT RUN {
## this may take a while (should get better area records)

county.areas <- 
    area.map(county.map, regions = county.map$names, sqmi = FALSE)
    
ancestry$density <- 
    with(ancestry, population / county.areas[as.character(county)])

mapplot(county ~ log(density), ancestry,
        map = county.map, border = NA,
        colramp = colorRampPalette(c("white", "black")))

# }
# NOT RUN {
# }