Faint lines appear on Swift Charts heat map

Question

I'm using Swift Charts to create a heat map as shown below and there are faint lines in the chart. In the screen shot, you can see the line in the Digital Color Meter window. I thought the lines were the grid lines of the chart but I made the X and Y axis hidden and the lines still appear. Any ideas on what is causing these lines to appear and how can I remove them?

import SwiftUI
import Charts

struct Point: Hashable, Identifiable {
    let id = UUID()
    let x: Int
    let y: Int
    let val: Float
}

struct Grid {
    
    let rows: Int
    let columns: Int
    var points = [Point]()
    
    init(rows: Int, columns: Int) {
        self.rows = rows
        self.columns = columns
        generateData()
    }
    
    mutating func generateData() {
        for i in 0..<rows {
            for j in 0..<columns {
                let v = Float.random(in: 0...1)
                let point = Point(x: j, y: i, val: v)
                points.append(point)
            }
        }
    }
}

struct ContentView: View {
    
    @State private var grid = Grid(rows: 10, columns: 10)
    
    var body: some View {
        Chart(grid.points) { point in
            RectangleMark(
                xStart: .value("xStart", point.x),
                xEnd: .value("xEnd", point.x + 1),
                yStart: .value("yStart", point.y),
                yEnd: .value("yEnd", point.y + 1)
            )
            .foregroundStyle(by: .value("Weight", point.val))
        }
        .chartYAxis(.hidden)
        .chartXAxis(.hidden)
        .chartForegroundStyleScale(range: Gradient(colors: [.yellow, .red]))
        .padding()
        .frame(minWidth: 400, minHeight: 400)
    }
}