A text description of a map drawn by the package, computed from the final
plot – its projection, its fill scale and the data it draws – so it stays
true when the plot is modified afterwards. Every map verb sets it as the
plot's alt text (labs(alt =)), which ggplot2::get_alt_text() returns and
which knitr, Quarto and Shiny can pass to a screen reader.
Details
The levels follow Lundgard & Satyanarayan (2022). Level 1 describes the construction: the kind of map, the projection, what the fill shows and how it is classified, and how many countries have data. Level 2 adds statistics: the three highest and three lowest countries, the share missing, and the continent with the highest median. Levels 3 and 4 – trends, and what the map means – are the author's to write; add them to the text this returns.
knitr does not ask ggplot2 for alt text when a chunk sets none, so in
R Markdown set it on the chunk, from a plot built in an earlier chunk:
fig.alt = countryatlas::map_alt_text(p).
References
Lundgard, A. & Satyanarayan, A. (2022). Accessible visualization via natural language descriptions: a four-level model of semantic content. IEEE Transactions on Visualization and Computer Graphics 28(1), 1073-1083. doi:10.1109/TVCG.2021.3114770
Examples
# \donttest{
snap <- countryatlas::world_snapshot$countries
p <- world_map(attach_geometry(snap), gdp_per_capita)
map_alt_text(p)
#> [1] "Choropleth map of GDP per capita (constant 2015 US$) (gdp_per_capita) by country, Equal Earth projection. Coloured in 5 quantile classes from 269 to 247K. 199 of 238 countries have data. Highest: Monaco (247K), Bermuda (117K), Luxembourg (104K). Lowest: Burundi (269), Afghanistan (374), Central African Republic (390). Europe has the highest median (22.7K). 16% of countries have no data."
ggplot2::get_alt_text(p)
#> [1] "Choropleth map of GDP per capita (constant 2015 US$) (gdp_per_capita) by country, Equal Earth projection. Coloured in 5 quantile classes from 269 to 247K. 199 of 238 countries have data. Highest: Monaco (247K), Bermuda (117K), Luxembourg (104K). Lowest: Burundi (269), Afghanistan (374), Central African Republic (390). Europe has the highest median (22.7K). 16% of countries have no data."
# }