Swift Charts - Is there a way query the y value of a LineMark at a specific x value?

Question

So I have a chart displaying line data at equal x intervals, with varying y values - something like this:

let ints = Array([0...10])
Chart {
    ForEach(ints) { int in
        let someRandomYValue = Int.random()
        LineMark(x: int, y: someRandomYValue, series: "Line")
    }
}

And I need to put an overlay exactly in the middle of two consecutive points. The overlay needs to touch the line.

So I need the x and the y value to place the overlay.

It's easy enough to get the x value - I just need the split the difference between my two x values. But I can't figure out how to get the y value.

Is there any way to "query" the y value of a line mark at specified x value? Maybe something like this:

let queriedY = LineMark(series: "Line").yValue(at: 8.5)

Answer 1

You can use .chartOverlay which gives you a chartProxy that can be queried for the position of chart values.

Here is an example:

struct ContentView: View {

    let data: [(Int, Int)] = {
        (0...15).map { ($0, Int.random(in: 0...50)) }
    }()

    @State private var selectedX = 6
    
    var body: some View {
        
        VStack {
            Chart {
                ForEach(data.indices, id: \.self) { i in
                    let (x,y) = data[i]
                    LineMark(
                        x: .value("x", x),
                        y: .value("y", y)
                    )
                }
            }
            .frame(height: 300)
            .chartOverlay { proxy in
                
                let pos1 = proxy.position(for: (x: selectedX, y: data[selectedX].1)) ?? .zero
                let pos2 = proxy.position(for: (x: selectedX+1, y: data[selectedX+1].1)) ?? .zero

                Text("x: \(selectedX), y: \(data[selectedX].1)")
                    .padding()
                    .background(.gray.opacity(0.5))
                    .position(x: (pos1.x + pos2.x)/2 , y: (pos1.y+pos2.y)/2)
            }
         
            Stepper("", value: $selectedX, in: 0...data.count-1)
            
        }
        .padding()
    }
}