Area chart
A line chart whose region between the curve and the baseline is filled. Use for magnitude-over-time visualisations where the area under the curve carries meaning.
Demo — NASA's share of the US federal budget, 1962–1972
The demo plots NASA spending as a percentage of the US federal
budget through the Apollo era. The 1966 peak (~4.4 %) and the
sharp post-Apollo wind-down to <2 % by 1972 tell the rise-and-fall
of the moon program in one shape. Reveal eases in over 1.5 s
with Ease::OutCubic.
Source: Budget of NASA — Wikipedia
Public surface
let plot = Plot::new(daily)
.mark(Mark::Area { interpolation: Interpolation::Linear })
.encode(plot::x("date", ScaleKind::Band))
.encode(plot::y("value", ScaleKind::Linear))
.encode(plot::color("region"));Convex-quad-per-segment emission
wisp's draw_polygon is convex-only in v1 — it fan-triangulates
from vertex 0, which produces overlapping triangles when the
input polygon is non-convex. A typical area chart polygon
(line + baseline) is non-convex whenever the line bends.
The renderer sidesteps this by emitting one convex
quadrilateral per segment: (x0, baseline) → (x1, baseline) → (x1, y1) → (x0, y0). Each quad is always convex, so the fan
triangulation is correct. Visually identical to one big polygon;
costs one extra primitive per segment.
Interpolation modes
| Mode | Quad shape |
|---|---|
Linear | Slanted top edge connecting (x0, y0) → (x1, y1) |
Step | Flat top edge at y0 from x0 to x1 (step bar) |
Pairing with Color encoding
A Color encoding splits rows into one series per category and
emits one polygon stream per series. Without an explicit
back-to-front render order today, overlapping areas can occlude
each other; future tickets will add z-ordering by series area.