class GradientMaskLineRenderer: LineChartRenderer {
  override init(dataProvider: LineChartDataProvider, animator: Animator, viewPortHandler: ViewPortHandler) {
    super.init(dataProvider: dataProvider, animator: animator, viewPortHandler: viewPortHandler)
  }

  override func drawFilledPath(context: CGContext, path: CGPath, fill: Fill, fillAlpha: CGFloat) {
    let rect = viewPortHandler.contentRect

    let colorSpace = CGColorSpaceCreateDeviceGray()
    let bitmapContext = CGContext(
      data: nil,
      width: Int(rect.size.width),
      height: Int(rect.size.height),
      bitsPerComponent: 8,
      bytesPerRow: Int(rect.size.width),
      space: colorSpace,
      bitmapInfo: CGImageAlphaInfo.none.rawValue
    )!

    let colors = [UIColor.white.cgColor, UIColor.black.cgColor] as CFArray
    let gradient = CGGradient(colorsSpace: colorSpace, colors: colors, locations: nil)!

    bitmapContext.drawLinearGradient(
      gradient,
      start: CGPoint(x: 0, y: 0),
      end: CGPoint(x: 0, y: rect.size.height),
      options: [.drawsAfterEndLocation, .drawsBeforeStartLocation]
    )

    let mask = bitmapContext.makeImage()!
    context.clip(to: rect, mask: mask)

    super.drawFilledPath(context: context, path: path, fill: fill, fillAlpha: fillAlpha)
  }
}