Charts: Bar Chart view group in Swift

Error : cannot invoke initializer for type with an argument list of type !!!

override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
let months = ["Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec"]
let unitsSold = [20.0, 4.0, 6.0, 3.0, 12.0, 16.0, 4.0, 18.0, 2.0, 4.0, 5.0, 4.0]
let unitsSold2 = [25.0, 2.0, 4.0, 10.0, 12.0, 16.0, 4.0, 18.0, 2.0, 4.0, 5.0, 4.0]
setChart(months, values: unitsSold, values2: unitsSold2)
}

func setChart(dataPoints: [String], values: [Double], values2: [Double]) {
    barChartView.noDataText = "You need to provide data for the chart."
    var dataEntries1: [BarChartDataEntry] = []
    var dataEntries2: [BarChartDataEntry] = []

    for i in 0..<dataPoints.count {
        let dataEntry1 = BarChartDataEntry(value: values[i], xIndex: i)

        let dataEntry2 = BarChartDataEntry(value: values2[i], xIndex: i)
        dataEntries1.append(dataEntry1)
        dataEntries2.append(dataEntry2)
    }

    let chartDataSet1 = BarChartDataSet(yVals: dataEntries1, label: "Units Sold")
    let chartDataSet2 = BarChartDataSet(yVals: dataEntries2, label: "Units Sold 2")
    var dataSet : [BarChartDataSet] = [BarChartDataSet]()
    dataSet.append(chartDataSet1)
    dataSet.append(chartDataSet2)


    let chartData = BarChartData(xVals: dataPoints, dataSet: dataSet) ->> 
    ->> Error : cannot invoke initializer for type with an argument list of type !!!


    chartData.groupSpace = 1
    barChartView.data = chartData
}