Charts: 如何通过虚线绘制 gridBackground

chartView.(rightAxis/leftAxis/xAxis).grid*

定义如下：

    public var axisLineColor = NSUIColor.grayColor()
    public var axisLineWidth = CGFloat(0.5)
    public var axisLineDashPhase = CGFloat(0.0)
    public var axisLineDashLengths: [CGFloat]!

    public var gridColor = NSUIColor.grayColor().colorWithAlphaComponent(0.9)
    public var gridLineWidth = CGFloat(0.5)
    public var gridLineDashPhase = CGFloat(0.0)
    public var gridLineDashLengths: [CGFloat]!
    public var gridLineCap = CGLineCap.Butt

    public var drawGridLinesEnabled = true
    public var drawAxisLineEnabled = true

leftAxis.gridLineDashLengths = @[@2];

我忘了设置这个属性。