Bubble chart
A bubble chart is a scatterplot with a third magnitude encoded as marker size — the canonical multi-channel "ah ha" plot.
The demo plots Gapminder 2007: GDP per capita (PPP) × life expectancy × population across 13 countries, coloured by continent. Hans Rosling made this dataset famous in his 2006 TED talk "The best stats you've ever seen", animating the same encoding from 1800 → present to show every country's simultaneous arc through rising income + rising lifespan.
Source: Gapminder Foundation — Wikipedia
Public surface
let plot = Plot::new(countries)
.mark(Mark::Point { shape: PointShape::Circle })
.encode(plot::x("gdp", ScaleKind::Log))
.encode(plot::y("life_expectancy", ScaleKind::Linear))
.encode(plot::size("population").size_mapping(SizeMapping::Area))
.encode(plot::color("continent"));Area vs radius mapping
Always use SizeMapping::Area for magnitude data. Radius
mapping is visually misleading: a 4× value renders 16× larger
because area = πr². Area mapping preserves the perceptual link
between value and visible bubble size.
SizeMapping | Behaviour | When to use |
|---|---|---|
Area (default) | sqrt(scaled value) → radius. 4× value → 4× visible bubble. | Magnitudes, populations, totals |
Radius | Scaled value → radius directly. | When the value is already a length/distance |
Multi-encoding read
A typical bubble chart uses 4 channels at once: X, Y, Size,
Color. Combined with Plot::legend,
the reader sees the Color legend automatically; future tickets
will add an explicit Size legend overlay for the third dimension.