How does a histogram work for the different channels in vImageHistogramCalculation_Planar8

I am attempting to calculate a histogram for the Y channel in a kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange image buffer. When I use vImageHistogramCalculation_Planar8 I pass in a reference to only a single histogram.

How do I know which channel is being used to create the histogram? What would I do if I wanted to read all channels?

Also open to critiques of the code sample.

extension ViewController: AVCaptureVideoDataOutputSampleBufferDelegate {
    func captureOutput(_ captureOutput: AVCaptureOutput!,
                       didOutputSampleBuffer sampleBuffer: CMSampleBuffer!,
                       from connection: AVCaptureConnection!) {

        let imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)!
        CVPixelBufferLockBaseAddress(imageBuffer, CVPixelBufferLockFlags(rawValue: 0))

        let height = CVPixelBufferGetHeight(imageBuffer)
        let width = CVPixelBufferGetWidth(imageBuffer)
        let bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer)
        let pixelBuffer = CVPixelBufferGetBaseAddress(imageBuffer)

//        let format = CVPixelBufferGetPixelFormatType(imageBuffer)
//        print("format: \(format)")

        ///kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange = '420v'

        var vBuffer = vImage_Buffer()
        vBuffer.data = pixelBuffer
        vBuffer.rowBytes = bytesPerRow
        vBuffer.width = vImagePixelCount(width)
        vBuffer.height = vImagePixelCount(height)

        let luma = [UInt](repeating: 0, count: 256)

        let lumaHist = UnsafeMutablePointer<vImagePixelCount>(mutating: luma)

        vImageHistogramCalculation_Planar8(&vBuffer, lumaHist, UInt32(kvImageNoFlags))

        CVPixelBufferUnlockBaseAddress(imageBuffer, CVPixelBufferLockFlags(rawValue: 0))
    }
}