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Line chart

A line chart connects rows of a DataFrame in order with a stroked polyline. Multi-series support comes via a Color encoding: each distinct category becomes its own line.

Demo — US unemployment rate, 1929–1941

The demo plots annual US unemployment through the Great Depression and into early World War II. The peak (24.9 % in 1933) is the worst single year on record; the 1937–38 secondary peak is the "Roosevelt Recession" after premature monetary tightening. The chart fades in over 1.5 s with Ease::OutCubic — a subtle "the chart is rendering itself" cue rather than a hard appear.

Source: Unemployment in the United States — Wikipedia

Public surface

let plot = Plot::new(daily)
    .mark(Mark::Line {
        interpolation: Interpolation::Linear,
        marker: Some(PointStyle::Circle),
    })
    .encode(plot::x("date", ScaleKind::Band))
    .encode(plot::y("value", ScaleKind::Linear))
    .encode(plot::color("metric"));

Mark variants

VariantWhen to use
Mark::Line { Linear, None }Standard time-series / continuous-x lines
Mark::Line { Step, None }Monotonic step series (quarterly milestones, billing tiers)
Mark::Line { *, Some(Circle) }Sparse data — readers spot individual points

Interpolation

Info

Interpolation::Linear connects (x₁, y₁) → (x₂, y₂) directly. Interpolation::Step inserts an L-shaped joint at (x₂, y₁), producing horizontal-then-vertical segments. Each step segment doubles the primitive count vs Linear.

Multi-series via Color encoding

When a Color encoding is present, the renderer splits rows by the color column's value and emits one polyline per series. Each series picks a palette colour from the theme. Use Plot::legend to auto-build a matching legend.

Theme integration

Theme fieldDrives
theme.plot.line_width_pxStroke thickness of every line
theme.plot.line_marker_radius_pxRadius of PointStyle::Circle markers
theme.palettePer-series stroke colour

Coordinate convention

Warning

For the Band X scale (categorical x), each row's x position is the centre of its band. For continuous X (Linear scale on a numeric column), the x position is the scale's map(value). The fundamental NDC flip is the same as bars — see Axes for the coordinate-convention note.