Agent skill

swift-charts

Implement, review, or improve data visualizations using Swift Charts. Use when building bar, line, area, point, pie, or donut charts; when adding chart selection, scrolling, or annotations; when plotting functions with vectorized BarPlot, LinePlot, AreaPlot, or PointPlot; when customizing axes, scales, legends, or foregroundStyle grouping; or when creating specialized visualizations like heat maps, Gantt charts, stacked/grouped bars, sparklines, or threshold lines.

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SKILL.md

Swift Charts

Build data visualizations with Swift Charts targeting iOS 26+. Compose marks inside a Chart container, configure axes and scales with view modifiers, and use vectorized plots for large datasets.

See references/charts-patterns.md for extended patterns, accessibility, and theming guidance.

Contents

  • Workflow
  • Chart Container
  • Mark Types
  • Axis Customization
  • Scale Configuration
  • Foreground Style and Encoding
  • Selection (iOS 17+)
  • Scrollable Charts (iOS 17+)
  • Annotations
  • Legend
  • Vectorized Plots (iOS 18+)
  • Common Mistakes
  • Review Checklist
  • 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, or SectorMark.
  3. Wrap marks in a Chart container.
  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+ 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+)

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: String?

// 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 (pie/donut)
.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: .center, spacing: 10) // Position
.chartLegend(position: .bottom) {                                // Custom
    HStack(spacing: 16) {
        ForEach(categories, id: \.self) { cat in
            Label(cat, systemImage: "circle.fill").font(.caption)
        }
    }
}

Vectorized Plots (iOS 18+)

Use for large datasets (1000+ points). Accept entire collections or functions.

swift
// Data-driven
Chart {
    BarPlot(sales, x: .value("Month", \.month), y: .value("Revenue", \.revenue))
        .foregroundStyle(\.barColor)
}

// Function plotting: y = f(x)
Chart {
    LinePlot(x: "x", y: "y", domain: -5...5) { x in sin(x) }
}

// Parametric: (x, y) = f(t)
Chart {
    LinePlot(x: "x", y: "y", t: "t", domain: 0...(2 * .pi)) { t in
        (x: cos(t), y: sin(t))
    }
}

Apply KeyPath-based modifiers before simple-value modifiers:

swift
BarPlot(data, x: .value("X", \.x), y: .value("Y", \.y))
    .foregroundStyle(\.color)    // KeyPath first
    .opacity(0.8)                // Value modifier second

Common Mistakes

1. Using ObservableObject instead of @Observable

swift
// WRONG
class ChartModel: ObservableObject {
    @Published var data: [Sale] = []
}
struct ChartView: View {
    @StateObject private var model = ChartModel()
}

// CORRECT
@Observable class ChartModel {
    var data: [Sale] = []
}
struct ChartView: View {
    @State private var model = ChartModel()
}

2. Missing series parameter for multi-line charts

swift
// WRONG -- all points connect into one line
Chart {
    ForEach(allCities) { item in
        LineMark(x: .value("Date", item.date), y: .value("Temp", item.temp))
    }
}

// CORRECT -- separate lines per city
Chart {
    ForEach(allCities) { item in
        LineMark(x: .value("Date", item.date), y: .value("Temp", item.temp))
            .foregroundStyle(by: .value("City", item.city))
    }
}

3. Too many SectorMark slices

swift
// WRONG -- 20 tiny sectors are unreadable
Chart(twentyCategories, id: \.name) { item in
    SectorMark(angle: .value("Value", item.value))
}

// CORRECT -- group into top 5 + "Other"
Chart(groupedData, id: \.name) { item in
    SectorMark(angle: .value("Value", item.value))
        .foregroundStyle(by: .value("Category", item.name))
}

4. Missing scale domain when zero-baseline matters

swift
// WRONG -- axis starts at ~95; small changes look dramatic
Chart(data) {
    LineMark(x: .value("Day", $0.day), y: .value("Score", $0.score))
}

// CORRECT -- explicit domain for honest representation
Chart(data) {
    LineMark(x: .value("Day", $0.day), y: .value("Score", $0.score))
}
.chartYScale(domain: 0...100)

5. Static foregroundStyle overriding data encoding

swift
// WRONG -- static color overrides by-value encoding
BarMark(x: .value("X", item.x), y: .value("Y", item.y))
    .foregroundStyle(by: .value("Category", item.category))
    .foregroundStyle(.blue)

// CORRECT -- use only the data encoding
BarMark(x: .value("X", item.x), y: .value("Y", item.y))
    .foregroundStyle(by: .value("Category", item.category))

6. Individual marks for 10,000+ data points

swift
// WRONG -- creates 10,000 mark views; slow
Chart(largeDataset) { item in
    PointMark(x: .value("X", item.x), y: .value("Y", item.y))
}

// CORRECT -- vectorized plot (iOS 18+)
Chart {
    PointPlot(largeDataset, x: .value("X", \.x), y: .value("Y", \.y))
}

7. Fixed chart height breaking Dynamic Type

swift
// WRONG -- clips axis labels at large text sizes
Chart(data) { ... }
    .frame(height: 200)

// CORRECT -- adaptive sizing
Chart(data) { ... }
    .frame(minHeight: 200, maxHeight: 400)

8. KeyPath modifier after value modifier on vectorized plots

swift
// WRONG -- compiler error
BarPlot(data, x: .value("X", \.x), y: .value("Y", \.y))
    .opacity(0.8)
    .foregroundStyle(\.color)

// CORRECT -- KeyPath modifiers first
BarPlot(data, x: .value("X", \.x), y: .value("Y", \.y))
    .foregroundStyle(\.color)
    .opacity(0.8)

9. Missing accessibility labels

swift
// WRONG -- VoiceOver users get no context
Chart(data) {
    BarMark(x: .value("Month", $0.month), y: .value("Sales", $0.sales))
}

// CORRECT -- add per-mark accessibility
Chart(data) { item in
    BarMark(x: .value("Month", item.month), y: .value("Sales", item.sales))
        .accessibilityLabel("\(item.month)")
        .accessibilityValue("\(item.sales) units sold")
}

Review Checklist

  • Data model uses Identifiable or chart uses id: key path
  • Model uses @Observable with @State, not ObservableObject
  • Mark type matches goal (bar=comparison, line=trend, sector=proportion)
  • Multi-series lines use series: parameter or .foregroundStyle(by:)
  • Axes configured with appropriate labels, ticks, and grid lines
  • Scale domain set explicitly when zero-baseline matters
  • Pie/donut limited to 5-7 sectors; small values grouped into "Other"
  • Selection binding type matches axis data type (Date? for date axis)
  • Scrollable charts set .chartXVisibleDomain(length:) for viewport
  • Vectorized plots used for datasets exceeding 1000 points
  • KeyPath modifiers applied before value modifiers on vectorized plots
  • Accessibility labels added to marks for VoiceOver
  • Chart tested with Dynamic Type and Dark Mode
  • Legend visible and positioned, or intentionally hidden
  • Ensure chart data model types are Sendable; update chart data on @MainActor

References

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