ReactorCharting Chart Authoring Guide

ReactorCharting is a C# port of D3.js that renders charts as Reactor Element trees on WinUI3 Canvas. Charts are pure functions: data => Element. The goal is a single return D3Canvas(...) expression per chart with no imperative mutation.

Imports

Every chart file needs these:

using Microsoft.UI.Reactor.Core;
using Microsoft.UI.Reactor.Charting.D3;
using Microsoft.UI.Reactor.Charting;
using static Microsoft.UI.Reactor.Charting.D3;   // D3Rect, D3Circle, D3LinePath, Brush, Gray, etc.
using static Microsoft.UI.Reactor.Factories;              // Text, Canvas, ScrollView, etc.

The Ideal Shape

The best charts follow this structure — one return, one collection expression, data transforms via LINQ, all primitives from the DSL:

return D3Canvas(W, H,
    [.. D3Grid(ys, left, pw),
     .. D3Axes(xs, ys, left, top, pw, ph),
     .. data.Select(d => D3Circle(xs.Map(d.x), ys.Map(d.y), r: 4)
         with { Fill = fill, Stroke = stroke }),
     D3Text(left, 6, "Title", fontSize: 14, foreground: Gray(40))]
);

Key elements: D3Canvas wraps everything, [.. spread] composes flat element lists, .Select() / .SelectMany() map data to elements, with { } sets visual properties.

Scales

Scales map data values to pixel positions. Create them, then call .Map(value) in accessors.

// Continuous: numeric domain -> pixel range
var xs = new LinearScale([0, 100], [left, left + pw]).Nice();
var ys = new LinearScale([0, maxVal], [top + ph, top]).Nice();  // Y is inverted (0 at bottom)

// Categorical: discrete labels -> bands
var band = BandScale.Create(labels)
    .SetRange(0, plotW)
    .SetPaddingInner(0.2)
    .SetPaddingOuter(0.1);
double x = left + band.Map("Jan");
double barWidth = band.Bandwidth;

Always call .Nice() on Y scales to get clean tick numbers. The Y range goes from [bottom, top] (inverted) so larger values map to smaller Y pixel values.

DSL Primitives

All return Reactor Elements. Customize with with { } for Fill, Stroke, etc.

D3Rect(x, y, width, height)                          // Rectangle
D3Circle(cx, cy, r: 4)                               // Circle (name r: when literal)
D3Line(x1, y1, x2, y2)                               // Line segment
D3Text(x, y, "label", fontSize: 11, foreground: brush) // Left-aligned text
D3TextRight(x, y, "label", width: 50, fontSize: 10)  // Right-aligned (for Y axis labels)
D3TextCenter(x, y, "label", width: 40, fontSize: 10) // Center-aligned

Use named parameters for fontSize:, foreground:, r:, width:, alpha:, opacity: — bare numeric literals are ambiguous without names.

Generator Helpers

Use the one-shot DSL helpers instead of constructing generator instances:

// Line from data -> PathElement (one expression, no intermediate variables)
D3LinePath(data, x: d => xs.Map(d.x), y: d => ys.Map(d.y),
    stroke: Brush(Palette[0]), strokeWidth: 2, curve: D3Curve.MonotoneX)

// Area from data -> PathElement
D3AreaPath(data, x: d => xs.Map(d.x), y0: d => ys.Map(0), y1: d => ys.Map(d.y),
    fill: Brush(Palette[0], opacity: 0.3))

// Arc sector -> PathElement (for pie slices, sunbursts)
D3ArcPath(startAngle, endAngle, cx, cy,
    innerRadius: 80, outerRadius: 150, fill: sliceBrush)

// Pie slices from data -> Element[] (simple case, no labels)
D3Pie(data, value: d => d.Value, cx, cy,
    outerRadius: 150, innerRadius: 0, sort: false)

Always use named parameters x:, y:, y0:, y1:, value: on these helpers — three consecutive lambdas are unreadable without names.

For pie charts with labels, use the static one-shot + D3ArcPath:

var arcs = PieGenerator.Generate(data, value: d => d.Value, sort: false, padAngle: 0.02);
arcs.SelectMany((a, i) => new Element[]
{
    D3ArcPath(a.StartAngle, a.EndAngle, cx, cy, outerRadius: 150, fill: Brush(Palette[i])),
    D3Text(cx + lx, cy + ly, a.Data.Name, fontSize: 10, foreground: brush),
})

Composite Helpers

D3Grid(yScale, left, plotWidth)                    // Horizontal grid lines -> Element[]
D3Axes(xScale, yScale, left, top, width, height)   // Axis lines + tick labels -> Element[]
D3Legend(x, y, items.Select((label, brush) => (label, brush)))  // Vertical legend -> Element[]
D3Link(x1, y1, x2, y2, stroke: brush)              // Bezier tree link -> PathElement

Color Helpers

Brush(Palette[0])                      // Categorical color from D3 Category10
Brush(Palette[i], opacity: 0.6)        // With opacity (always name opacity:)
Brush("#2ca02c")                       // From hex string
Gray(100)                              // Grayscale
Gray(100, alpha: 180)                  // Grayscale with alpha (always name alpha:)

Palette is D3Color.Category10 — 10 categorical colors. Access with Palette[i % Palette.Length] to avoid index errors.

Data-to-Elements Patterns

Simple projection — .Select()

When each data item produces one element:

.. points.Select(p => (Element)(D3Circle(xs.Map(p.x), ys.Map(p.y), r: 4)
    with { Fill = fill }))

The (Element) cast is sometimes needed for spread in collection expressions — this is a C# limitation, not a DSL issue.

Multi-element projection — .SelectMany()

When each data item produces multiple elements (e.g., bar + label):

.. items.SelectMany(item => new Element[]
{
    D3Rect(x, y, w, h) with { Fill = fill },
    D3Text(x + 4, y + 3, item.Name, fontSize: 9, foreground: Gray(20)),
})

Query syntax — from/let/select

When you need 3+ intermediate values before the final element, use query syntax instead of a multi-line lambda:

.. (from t in candles.Select((c, i) => (c, i))
    let cx = xs.Map(t.i)
    let bullish = t.c.Close >= t.c.Open
    let brush = bullish ? bullBrush : bearBrush
    let bodyTop = ys.Map(Math.Max(t.c.Open, t.c.Close))
    let bodyH = Math.Max(ys.Map(Math.Min(t.c.Open, t.c.Close)) - bodyTop, 1)
    from el in new Element[]
    {
        D3Line(cx, ys.Map(t.c.High), cx, ys.Map(t.c.Low)) with { Stroke = brush },
        D3Rect(cx - barW / 2, bodyTop, barW, bodyH) with { Fill = brush },
    }
    select el)

Keep .Select() for simple 1-line projections. Switch to query syntax at 3+ let bindings.

Stacked/grouped series

.. series.SelectMany((s, si) =>
{
    var fill = Brush(Palette[si]);
    return months.Select((month, j) =>
    {
        var pt = s.Points[j];
        double x = left + band.Map(month);
        return D3Rect(x, y1Screen, band.Bandwidth, height) with { Fill = fill };
    });
})

Things to Do

• Return a single D3Canvas(W, H, [...]) expression — the entire chart is one return statement
• Use [.. spread] syntax to compose flat element arrays: [.. grid, .. axes, .. bars, title]
• Use LINQ .Select() / .SelectMany() to map data to elements — no for-loops
• Use with { Fill = ..., Stroke = ... } to set visual properties on elements
• Use named parameters on calls with multiple double/brush args: fontSize:, foreground:, opacity:, alpha:, r:, x:, y:, y0:, y1:, value:, width:
• Use D3LinePath / D3AreaPath instead of constructing LineGenerator/AreaGenerator instances
• Use D3ArcPath instead of constructing ArcGenerator instances
• Use D3Grid / D3Axes / D3Legend instead of manually building grid lines, axis labels, or legends
• Use Palette[i % Palette.Length] to avoid index-out-of-range on category colors
• Use D3Extent.Extent(data, accessor) for finding min/max — not manual loops
• Use .Nice() on scales to get clean tick boundaries
• Pre-compute brushes outside the element expression when reused: var fill = Brush(Palette[0], opacity: 0.6);

Things to Avoid

• Don't construct generator instances (e.g., new ArcGenerator(), LineGenerator.Create(...)) when a DSL helper exists — use D3LinePath, D3AreaPath, D3ArcPath, D3Pie instead
• Don't use for-loops to build element arrays — use .Select(), .SelectMany(), or query syntax
• Don't use XamlHostElement or raw XAML — everything should go through the D3 DSL primitives
• Don't use .Set(tb => tb.TextAlignment = ...) — use D3TextCenter or D3TextRight instead
• Don't use Text(...).Foreground(...).Canvas(...) — use D3Text(x, y, text, fontSize:, foreground:) instead
• Don't create mutable state — no var elements = new List<Element>(); elements.Add(...) patterns
• Don't use Array.Empty<Element>() in ternary spreads for null paths — D3Path accepts null gracefully
• Don't duplicate generators — e.g., creating two identical AreaGenerators when one suffices; or creating a separate labelArc ArcGenerator just for centroids (use ArcGenerator.Centroid(...) static method instead)
• Don't use .Where().Select() chains that shift indices — either inline the filter or use query syntax with where before select to preserve the original index
• Don't write bare numeric literals for optional parameters — Gray(100, 180) is ambiguous; write Gray(100, alpha: 180)
• Don't use Microsoft.UI.Xaml.Media.SolidColorBrush(Microsoft.UI.Colors.White) — write Brush("#ffffff")
• Don't manually build legends with SelectMany producing (rect, text) pairs — use D3Legend(x, y, items)
• Don't manually build grid lines with tick iteration — use D3Grid(yScale, left, width)
• Don't manually build axis labels when D3Axes(xs, ys, ...) covers your case
• Don't use D3Color.Parse(...) then Brush(...) — just use Brush("#hex") directly