SwiftUI Chart Bar Graph with Side-by-Side Bars

Question

I"m try to display two sets of data on a SwiftUI Charts Bar Graph. I want to bars to display side-by-side (i.e. blue-green, blue-green for the sample image). However, what I'm getting is the two sets of bars displayed separately as shown in the image. What am I doing wrong? Here's my code: The structure of the data isn't fixed, so it can change if necessary.

struct ContentView: View {

var body: some View {
    let wheelDevelopmentArray = [
        (chainring: "one", eff:[75, 69, 64, 60, 56, 53, 47, 43, 39, 36, 32]),
        (chainring: "two", eff:[110, 101, 94, 88, 82, 77, 69, 63, 57, 53, 47])
    ]
    
    Chart(wheelDevelopmentArray, id: \.chainring) { wheels in
        ForEach(0..<wheels.eff.count, id: \.self) { i in
                    BarMark(
                        x: .value("gear", wheels.chainring),
                        y: .value("size", wheels.eff[i]),
                        width: 20
                    )
                    .foregroundStyle(by: .value("ChainRing", wheels.chainring))
                    .position(by: .value("ChainRing", i))
                }
            }
    .chartXAxis {
                AxisMarks (values: .stride (by: 1)) { value in
                    AxisValueLabel()
                }
            }
    .padding()
}

}