Theme — shared across every chart family

wisp_chart::Theme is the single entry point that themes any chart family consistently. Pick Theme::light() once and render a Gantt, a bar chart, a KPI card, or a sunburst against it — each chart family reads only from its own sub-theme plus the top-level shared fields, so adding a new chart never reshapes existing themes.

Shape

graph LR
    T[Theme]
    T --> bg[bg : Color]
    T --> tp[text_primary : Color]
    T --> tm[text_muted : Color]
    T --> pal[palette : OwnerPalette]
    T --> plot[plot : PlotTheme]
    T --> axis[axis : AxisTheme]
    T --> leg[legend : LegendTheme]
    T --> ind[indicator : IndicatorTheme]
    T --> gantt[gantt : GanttTheme]
    classDef shared fill:#7c2d12,stroke:#ea580c,color:#fed7aa
    classDef sub fill:#1e293b,stroke:#475569,color:#cbd5e1
    class bg,tp,tm,pal shared
    class plot,axis,leg,ind,gantt sub

Top row (orange) are the shared fields every chart family may read. The five sub-themes (slate) are each owned by a specific chart-family — theme.gantt.* is for Gantt, theme.plot.* / theme.axis.* / theme.legend.* for cartesian families, and so on.

Field inventory

PlotTheme:

• gridline_major — LineStyle (default #cccccc, 2 px). Major-tick gridlines on cartesian / heatmap charts.
• gridline_minor — LineStyle (default #e5e5e5, 1 px). Minor-tick gridlines, e.g. weeks within a month.

AxisTheme:

• tick_length_px — tick mark length in device pixels (default 5).
• tick_label_font_size — tick label font size (default 12).
• tick_density_hint — target number of ticks the auto-tick generator aims for (default 8).

LegendTheme:

• swatch_size_px — colour box / line / marker size (default 14).
• item_spacing_px — spacing between legend items (default 8).
• item_font_size — legend item font size (default 12).

IndicatorTheme:

• numeric_font_size — big-number font (default 32).
• delta_up — positive delta colour (default #27ae60 green).
• delta_down — negative delta colour (default #e74c3c red).
• delta_neutral — neutral colour (default #888888 muted).

GanttTheme:

• row_alt_bg — alternating row tint (default #fafafa).
• header_bg — header band background (default #f5f5f5).
• grid_week / grid_month — Gantt-specific aliases for the plot gridlines (default mirrors theme.plot.gridline_minor / gridline_major).
• bar_corner_radius — bar corner radius in pixels (default 6).
• bar_height — bar height in pixels (default 28).
• row_height — row height in pixels (default 44).
• gutter_width — left gutter width (default 180).
• header_height — header band height (default 60).

Customising

The common case is "use Theme::light() but tweak one knob":

#![allow(unused)]
fn main() {
let mut theme = wisp_chart::Theme::light();
theme.gantt.bar_corner_radius = 12.0;   // chunkier Gantt bars
theme.indicator.delta_up = wisp_chart::Color::from_hex("#10b981").unwrap();
}

Spreading a sub-theme onto a fresh Theme works too:

#![allow(unused)]
fn main() {
let theme = wisp_chart::Theme {
    gantt: wisp_chart::theme::GanttTheme {
        bar_corner_radius: 12.0,
        ..wisp_chart::Theme::light().gantt
    },
    ..wisp_chart::Theme::light()
};
}

Theme::dark() lands as a follow-on once the cartesian wave is visible enough that dark-mode contrast checks are worth doing.

Verified by

crates/wisp-chart/src/theme.rs has four tests:

1. light_theme_uses_white_bg — top-level bg defaults to white.
2. light_theme_gantt_dimensions_match_spec — the Gantt sub-theme preserves the pre-decomposition pixel sizes (28/44/180/60/6) byte-identically.
3. light_theme_populates_every_sub_theme — every sub-theme has non-zero defaults where a real number is required.
4. gantt_grid_lines_match_legacy_aliases — Gantt's grid_week / grid_month track the plot gridlines, matching the original flat-Theme shape.

Plus the chart-web snapshot test crates/wisp-chart-web/tests/render_gantt.rs continues to pass unchanged: the Gantt PNG is byte-identical after the refactor.