In a real-time plot, filling under a line plot is handled exactly the same as in the QCChart2D software. You set the fill flag of the ChartAttribute controlling the objects color. In the example below the fill flag is set because the fourth parameter in the ChartAttribute constructor specifies a fill color (Green, same as the line color, in this case). You must also specify a Fill base value, 0.0 in the example below.
ChartAttribute attrib2 = new ChartAttribute(Color.Green, 3, DashStyle.Solid, Color.Green);
SimpleLinePlot lineplot2 = new SimpleLinePlot(pTransform2, null, attrib2);
lineplot2.FillBaseValue = 0;
lineplot2.SetFastClipMode(ChartObj.FASTCLIP_X);
rtPlot2 = new RTSimpleSingleValuePlot(pTransform2, lineplot2, NASDAQChannel);
chartVu.AddChartObject(rtPlot2);