swift - iOS-charts 饼图在 Xcode 7.1 中崩溃

Question

我正在为 iOS 图表做一个关于 AppCoda 的教程，应用程序上的条形图部分工作正常，但饼图部分崩溃并出现错误：致命错误：在展开可选值时意外发现 nil。它在此命令 pieChartView.data = pieChartData 上崩溃。这是代码：

func setChart(dataPoints: [String], values: [Double]) {

    var dataEntries: [ChartDataEntry] = []
    for i in 0..<dataPoints.count {
        let dataEntry = ChartDataEntry(value: values[i], xIndex: i)
        dataEntries.append(dataEntry)
    }

    let pieChartDataSet = PieChartDataSet(yVals: dataEntries)

    let pieChartData = PieChartData(xVals: dataPoints, dataSet: pieChartDataSet)
    pieChartDataSet.label = ""
    pieChartView.data = pieChartData

    var colors: [UIColor] = []

     colors.append(UIColor.greenColor())
     colors.append(UIColor.blueColor())
     colors.append(UIColor.blackColor())

    pieChartDataSet.colors = colors
}

有人知道为什么吗？