Динамически раскрашивать легенду Swift Chart

Сначала я бы создал такой [String: Color], чтобы сопоставить имена предметов с соответствующими цветами:

@State private var colors: [String: Color] = [:]

Вы можете установить это в onChange:

// this assumes that each name only corresponds to one color
// there must not be two subjects with the same name but different colours
.onChange(of: entryObservations, initial: true) { _, newValue in
    colors = Dictionary(uniqueKeysWithValues: Set(newValue.map(\.subject)).map { ($0.name, $0.color) })
}

onChange требует, чтобы аргумент of: соответствовал Equatable, поэтому вам следует сделать это для всех задействованных структур - EntryObservation, Subject и SectionGroup.

Затем вы можете использовать chartForegroundStyleScale, который принимает параметр функции, чтобы сопоставить каждое имя с цветом, используя словарь colors.

.chartForegroundStyleScale { (name: String) in
    colors[name] ?? .clear
}

Я бы рекомендовал проделать то же самое с двумя разными категориями. Создайте два массива ChartData, где ChartData содержит все необходимые каждому BarMark данные, и обновите массивы в onChange. Таким образом, вам не нужно filter при каждом обновлении просмотра.

Минимальный воспроизводимый пример:

@State private var colors: [String: Color] = [:]
@State private var categorization = 1
var body: some View {
    Chart (entryObservations, id: \.time) {entryObservation in
        Plot {
            switch categorization {
            case 2:
                BarMark(x: .value("Section", entryObservation.sectionGroup.name), y: .value("Count", 1))
                    .foregroundStyle(by: .value("Subject", entryObservation.subject.name))
                    .cornerRadius(10)
                    .annotation(position: .top) {
                        Text("\(entryObservations.filter { $0.sectionGroup.name == entryObservation.sectionGroup.name }.count)")
                            .foregroundColor(Color.gray)
                            .font(.system(size: 12, weight: .bold))
                    }
            default:
                BarMark(x: .value("Subject", entryObservation.subject.name), y: .value("Count", 1))
                    .cornerRadius(10)
                    .foregroundStyle(by: .value("Subject", entryObservation.subject.name))
                    .annotation(position: .top) {
                        Text("\(entryObservations.filter { $0.subject.name == entryObservation.subject.name }.count)")
                            .foregroundColor(Color.gray)
                            .font(.system(size: 12, weight: .bold))
                    }
            }
        }
    }
    .padding()
    .chartLegend(.visible)
    .chartYAxisLabel("Count")
    .safeAreaInset(edge: .top) {
        VStack {
            Picker("Categorisation", selection: $categorization) {
                Text("Subject").tag(1)
                Text("Section").tag(2)
            }
            .pickerStyle(.segmented)
        }
    }
    .onChange(of: entryObservations, initial: true) { _, newValue in
        colors = Dictionary(uniqueKeysWithValues: Set(newValue.map(\.subject)).map { ($0.name, $0.color) })
    }
    .chartForegroundStyleScale { (name: String) in
        colors[name] ?? .clear
    }
}

let entryObservations: [EntryObservation] = [
    EntryObservation(subject: Subject(name: "Men", color: .blue), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 1000)),
    EntryObservation(subject: Subject(name: "Men", color: .blue), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 1300)),
    EntryObservation(subject: Subject(name: "Men", color: .blue), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 2000)),
    EntryObservation(subject: Subject(name: "Boys", color: .red), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 1900)),
    EntryObservation(subject: Subject(name: "Boys", color: .red), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 2200)),
    EntryObservation(subject: Subject(name: "Boys", color: .red), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 2500)),
    EntryObservation(subject: Subject(name: "Girls", color: .teal), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 2800)),
    EntryObservation(subject: Subject(name: "Girls", color: .teal), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 3100)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 3400)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 3700)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 4000)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 4300)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 4600)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "Mall"), time: Date(timeIntervalSince1970: 4900)),
    EntryObservation(subject: Subject(name: "Women", color: .mint), sectionGroup: SectionGroup(name: "School"), time: Date(timeIntervalSince1970: 5200))
    
]