swift-charts

What this skill does

# Swift Charts

Build data visualizations with Swift Charts targeting iOS 26+. Compose marks
inside `Chart` or `Chart3D`, configure axes and scales with view modifiers, and
use vectorized plots or 3D plots when the data calls for them.

See [references/charts-patterns.md](references/charts-patterns.md) for extended patterns, 3D charts, accessibility, and theming guidance.

## Contents

- [Workflow](#workflow)
- [Chart Container](#chart-container)
- [Mark Types](#mark-types)
- [Axis Customization](#axis-customization)
- [Scale Configuration](#scale-configuration)
- [Foreground Style and Encoding](#foreground-style-and-encoding)
- [Selection (iOS 17+)](#selection-ios-17)
- [Scrollable Charts (iOS 17+)](#scrollable-charts-ios-17)
- [Annotations](#annotations)
- [Legend](#legend)
- [Vectorized Plots (iOS 18+)](#vectorized-plots-ios-18)
- [3D Charts (iOS 26+)](#3d-charts-ios-26)
- [Common Mistakes](#common-mistakes)
- [Review Checklist](#review-checklist)
- [References](#references)

## Workflow

### 1. Build a new chart

1. Define data as an `Identifiable` struct or use `id:` key path.
2. Choose mark type(s): `BarMark`, `LineMark`, `PointMark`, `AreaMark`,
   `RuleMark`, `RectangleMark`, `SectorMark`, or `SurfacePlot`.
3. Wrap 2D marks in `Chart`; use `Chart3D` only for real spatial or surface data.
4. Encode visual channels: `.foregroundStyle(by:)`, `.symbol(by:)`, `.lineStyle(by:)`.
5. Configure axes with `.chartXAxis` / `.chartYAxis`.
6. Set scale domains with `.chartXScale(domain:)` / `.chartYScale(domain:)`.
7. Add selection, scrolling, or annotations as needed.
8. For 1000+ 2D data points, use vectorized plots (`BarPlot`, `LinePlot`, etc.).

### 2. Review existing chart code

Run through the Review Checklist at the end of this file.

## Chart Container

```swift
// Data-driven init (single-series)
Chart(sales) { item in
    BarMark(x: .value("Month", item.month), y: .value("Revenue", item.revenue))
}

// Content closure init (multi-series, mixed marks)
Chart {
    ForEach(seriesA) { item in
        LineMark(x: .value("Date", item.date), y: .value("Value", item.value))
            .foregroundStyle(.blue)
    }
    RuleMark(y: .value("Target", 500))
        .foregroundStyle(.red)
}

// Custom ID key path
Chart(data, id: \.category) { item in
    BarMark(x: .value("Category", item.category), y: .value("Count", item.count))
}
```

## Mark Types

### BarMark (iOS 16+)

```swift
// Vertical bar
BarMark(x: .value("Month", item.month), y: .value("Sales", item.sales))

// Stacked by category (automatic when same x maps to multiple bars)
BarMark(x: .value("Month", item.month), y: .value("Sales", item.sales))
    .foregroundStyle(by: .value("Product", item.product))

// Horizontal bar
BarMark(x: .value("Sales", item.sales), y: .value("Month", item.month))

// Interval bar (Gantt chart)
BarMark(
    xStart: .value("Start", item.start),
    xEnd: .value("End", item.end),
    y: .value("Task", item.task)
)
```

### LineMark (iOS 16+)

```swift
// Single line
LineMark(x: .value("Date", item.date), y: .value("Price", item.price))

// Multi-series via foregroundStyle encoding
LineMark(x: .value("Date", item.date), y: .value("Temp", item.temp))
    .foregroundStyle(by: .value("City", item.city))
    .interpolationMethod(.catmullRom)

// Multi-series with explicit series parameter
LineMark(
    x: .value("Date", item.date),
    y: .value("Price", item.price),
    series: .value("Ticker", item.ticker)
)
```

### PointMark (iOS 16+)

```swift
PointMark(x: .value("Height", item.height), y: .value("Weight", item.weight))
    .foregroundStyle(by: .value("Species", item.species))
    .symbol(by: .value("Species", item.species))
    .symbolSize(100)
```

### AreaMark (iOS 16+)

```swift
// Stacked area
AreaMark(x: .value("Date", item.date), y: .value("Sales", item.sales))
    .foregroundStyle(by: .value("Category", item.category))

// Range band
AreaMark(
    x: .value("Date", item.date),
    yStart: .value("Min", item.min),
    yEnd: .value("Max", item.max)
)
.opacity(0.3)
```

### RuleMark (iOS 16+)

```swift
RuleMark(y: .value("Target", 9000))
    .foregroundStyle(.red)
    .lineStyle(StrokeStyle(dash: [5, 3]))
    .annotation(position: .top, alignment: .leading) {
        Text("Target").font(.caption).foregroundStyle(.red)
    }
```

### RectangleMark (iOS 16+)

```swift
RectangleMark(x: .value("Hour", item.hour), y: .value("Day", item.day))
    .foregroundStyle(by: .value("Intensity", item.intensity))
```

### SectorMark (iOS 17+)
Use `SectorMark` for strictly positive values; filter, aggregate, or explain zero/negative values outside the pie or donut.
```swift
// Pie chart
Chart(data, id: \.name) { item in
    SectorMark(angle: .value("Sales", item.sales))
        .foregroundStyle(by: .value("Category", item.name))
}

// Donut chart
Chart(data, id: \.name) { item in
    SectorMark(
        angle: .value("Sales", item.sales),
        innerRadius: .ratio(0.618),
        outerRadius: .inset(10),
        angularInset: 1
    )
    .cornerRadius(4)
    .foregroundStyle(by: .value("Category", item.name))
}
```

## Axis Customization

```swift
// Hide axes
.chartXAxis(.hidden)
.chartYAxis(.hidden)

// Custom axis content
.chartXAxis {
    AxisMarks(values: .stride(by: .month)) { value in
        AxisGridLine()
        AxisTick()
        AxisValueLabel(format: .dateTime.month(.abbreviated))
    }
}

// Multiple AxisMarks compositions (different intervals for grid vs. labels)
.chartXAxis {
    AxisMarks(values: .stride(by: .day)) { _ in AxisGridLine() }
    AxisMarks(values: .stride(by: .week)) { _ in
        AxisTick()
        AxisValueLabel(format: .dateTime.week())
    }
}

// Axis labels (titles)
.chartXAxisLabel("Time", position: .bottom, alignment: .center)
.chartYAxisLabel("Revenue ($)", position: .leading, alignment: .center)
```

## Scale Configuration

```swift
.chartYScale(domain: 0...100)                          // Explicit numeric domain
.chartYScale(domain: .automatic(includesZero: true))   // Include zero
.chartYScale(domain: 1...10000, type: .log)            // Logarithmic scale
.chartXScale(domain: ["Mon", "Tue", "Wed", "Thu"])     // Categorical ordering
```

## Foreground Style and Encoding

```swift
BarMark(...).foregroundStyle(.blue)                                    // Static color
BarMark(...).foregroundStyle(by: .value("Category", item.category))   // Data encoding
AreaMark(...).foregroundStyle(                                         // Gradient
    .linearGradient(colors: [.blue, .cyan], startPoint: .bottom, endPoint: .top)
)
```

## Selection (iOS 17+)

```swift
@State private var selectedDate: Date?
@State private var selectedRange: ClosedRange<Date>?
@State private var selectedAngle: Double?

// Point selection
Chart(data) { item in
    LineMark(x: .value("Date", item.date), y: .value("Value", item.value))
}
.chartXSelection(value: $selectedDate)

// Range selection
.chartXSelection(range: $selectedRange)

// Angular selection binds the plottable angle value; derive the category from ranges.
.chartAngleSelection(value: $selectedAngle)
```

## Scrollable Charts (iOS 17+)

```swift
Chart(dailyData) { item in
    BarMark(x: .value("Date", item.date, unit: .day), y: .value("Steps", item.steps))
}
.chartScrollableAxes(.horizontal)
.chartXVisibleDomain(length: 3600 * 24 * 7) // 7 days visible
.chartScrollPosition(initialX: latestDate)
.chartScrollTargetBehavior(
    .valueAligned(matching: DateComponents(hour: 0), majorAlignment: .page)
)
```

## Annotations

```swift
BarMark(x: .value("Month", item.month), y: .value("Sales", item.sales))
    .annotation(position: .top, alignment: .center, spacing: 4) {
        Text("\(item.sales, format: .number)").font(.caption2)
    }

// Overflow resolution
.annotation(
    position: .top,
    overflowResolution: .init(x: .fit(to: .chart), y: .padScale)
) { Text("Label") }
```

## Legend

```swift
.chartLegend(.hidden)                                           // Hide
.chartLegend(position: .bottom, alignment: .c