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", "jenks"),
  palette = NULL,
  n_bins = 5,
  alpha_range = c(0.15, 1),
  transform = c("rank", "log10", "identity"),
  background = "grey20",
  title = NULL,
  legend = NULL,
  projection = "equal_earth"
)

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 world_map()).

palette

Optional viridis palette name.

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 for the sf backend.

Value

A ggplot object.

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

See also

cartogram_map() and dorling_map() (the geometry-distorting answers to the same problem), world_map()

Examples

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

# }