protocol witness error in Playgrounds

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I'm importing SwiftUI, Foundation and Charts into an iOS app I'm writing in Swift in Xcode Playgrounds and am getting this error:

error: Couldn't look up symbols: protocol witness table for Foundation.Date : Charts.Plottable in Charts

the code looks like this in just two example files:

file 1, the view

import Foundation
import SwiftUI
import Charts
import PlaygroundSupport 
 
struct FirstChart_WithDates: View {
    private let data = ChartDateAndDoubleModel.mockData(months: 3)
    
    var body: some View {
        Chart(data) { item in
            BarMark(
                x: .value("Label", item.date, unit: .month),
                y: .value("Value", item.value)
            )
        }
        .padding()
        .aspectRatio(1, contentMode: .fit)
        .dynamicTypeSize(.accessibility1)
        
        ChartDateAndDoubleModelView(data: data)
    }
}

struct ChartDateAndDoubleModelView: View {
    var data: [ChartDateAndDoubleModel]
    
    var body: some View {
        VStack {
            HeaderRowView(texts: ["date", "value"])
            
            ForEach(data) { datum in
                HStack {
                    Text(datum.date.formatted(date: .abbreviated, time: .omitted))
                        .frame(maxWidth: .infinity)
                    // TODO: Format for 2 decimal places.
                    Text(datum.value, format: .number.precision(.fractionLength(2)))
                        .frame(maxWidth: .infinity)
                }
            }
        }
        .padding(.bottom, 8)
        .overlay(.quaternary, in: .rect(cornerRadius: 8).stroke())
        .padding()
    }
}

struct HeaderRowView: View {
    var texts: [String]

    var body: some View {
        HStack(spacing: 2.0) {
            ForEach(texts, id: \.self) { text in
                Text(text)
                    .padding(4)
                    .frame(maxWidth: .infinity)
                    .background(.quaternary)
            }
        }
        .font(.headline)
        .mask(UnevenRoundedRectangle(topLeadingRadius: 8, topTrailingRadius: 8))
    }
}

and file 2, the model:

import Foundation
import SwiftUI
import Charts


//  ChartDateAndDoubleModel.swift
//  
//
//  Created by Michael Isbell on 2/10/26.
//


public struct ChartDateAndDoubleModel: Identifiable {
    public let id = UUID()
    public let date: Date
    public let value: Double
}

public extension ChartDateAndDoubleModel {
     static func mockData(months: Int = 5) -> [ChartDateAndDoubleModel] {
        
        var data: [ChartDateAndDoubleModel] = []
        let calendar = Calendar(identifier: .gregorian)
        let currentDate = Date()
        
        for month in 0..<months {
            //add month to current date and append to data
            data.append(
                ChartDateAndDoubleModel(
                    date: calendar.date(byAdding: .month, value: month, to: currentDate)!,
                    value: Double.random(in: 0...1000)
                                               )
            )
        }
        return data
    }
}

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