Value-by-alpha: equalise a rate by its denominator

A choropleth where colour carries the value and opacity carries an equalising variable (usually population), over a neutral background. It is the answer to the small-number problem – a rate computed over eleven thousand people shouts as loudly as one computed over a billion – and unlike a cartogram it solves it without distorting geometry, which is the main objection to cartograms. Roth, Woodruff & Johnson (2010) introduced it for exactly this purpose.

Usage

value_by_alpha_map(
  data,
  value,
  equalize,
  style = c("quantile", "continuous", "binned", "equal", "jenks", "fisher", "headtails",
    "sd", "fixed"),
  palette = NULL,
  n_bins = 5,
  breaks = NULL,
  midpoint = NULL,
  alpha_range = c(0.15, 1),
  transform = c("rank", "log10", "identity"),
  background = "grey20",
  title = NULL,
  legend = NULL,
  projection = "equal_earth",
  footnote = "auto",
  small_states = c("auto", "none", "dots"),
  small_area_km2 = 1000
)

Arguments

data: A map-ready frame (polygon or sf).
value: The value column, carried by colour (unquoted).
equalize: The equalising column, carried by opacity (unquoted) – population, total counts, or whatever denominator the rate was built on.
style: Classification for the colour channel, as in world_map() (any of its styles but "categorical").
palette, breaks, midpoint: As in world_map().
n_bins: Number of colour bins for the binned styles.
alpha_range: Minimum and maximum opacity (default c(0.15, 1)).
transform: Transform applied to equalize before it is mapped to opacity: "rank" (default, robust to the extreme skew of population), "log10" or "identity".
background: Colour behind the countries, which shows through where opacity is low (default a dark neutral).
title, legend: Optional plot title and legend title.
projection: Projection, as in world_map().
footnote, small_states, small_area_km2: As in world_map(). A small state drawn as a point takes its opacity from equalize too.

Value

A ggplot object.

Backend

Either backend, from the frame: an sf frame draws through coord_sf(), a polygon frame on the polygon backend, both in projection (Equal Earth by default).

References

Roth, R. E., Woodruff, A. W. & Johnson, Z. F. (2010). Value-by-alpha maps: an alternative technique to the cartogram. The Cartographic Journal 47(2), 130-140. doi:10.1179/000870409X12488753453372

Examples

# \donttest{
snap <- countryatlas::world_snapshot$countries
attach_geometry(snap, geometry = "polygon") |>
  value_by_alpha_map(gdp_per_capita, population)

# }