WLColor-Transparent-fill-comes-out-black

CODE:
using WealthLab.Backtest;
using WealthLab.Core;
using WealthLab.Indicators;

namespace WealthScript3 
{
   public class MyStrategy : UserStrategyBase
   {
      public override void Initialize(BarHistory bars)
      {
         IndicatorBase roc = ROC.Series(bars.Close,20);
         PlotTimeSeriesOscillator(roc*2.0, roc.Description, "upperPane", 0.0, 0.0, WLColor.Brown.SetAlpha(140), WLColor.Yellow, WLColor.Yellow, 150);
         PlotTimeSeriesOscillator(roc, roc.Description, "upperPane", 0.0, 0.0, WLColor.SteelBlue, WLColor.Salmon, WLColor.Green, 250);
         
         //now with WLColor.Transparent fill, which comes out black
         PlotTimeSeriesOscillator(roc*2.0, roc.Description, "lowerPane", 0.0, 0.0, WLColor.Brown.SetAlpha(140), WLColor.Transparent, WLColor.Yellow, 150);
         PlotTimeSeriesOscillator(roc, roc.Description, "lowerPane", 0.0, 0.0, WLColor.SteelBlue, WLColor.Salmon, WLColor.Green, 250);
      }

      public override void Execute(BarHistory bars, int idx) { }
   }
}