GraphicsGradient.linear(num startX, num startY, num endX, num endY)

Source 

GraphicsGradient.linear(this.startX, this.startY, this.endX, this.endY) :
    this.startRadius = 0.0,
    this.endRadius = 0.0,
    this.kind = "linear",
    this.colorStops = new List<GraphicsGradientColorStop>();