如何创建虚线c#图形

我试图添加一个虚线的图形，但它一直出来作为实线。我怎样才能解决这个问题呢?

static public NPlot.Bitmap.PlotSurface2D getDashlineGraph(int itemID, int width, int height)
{
    NPlot.Bitmap.PlotSurface2D plot = new NPlot.Bitmap.PlotSurface2D(width, height);
    plot.BackColor = Color.WhiteSmoke;
    plot.SmoothingMode = System.Drawing.Drawing2D.SmoothingMode.HighQuality;
    LinePlot TotaldashLine = new LinePlot(straightline, timer);     
    float[] pattern = { 5.0f, 10.0f };
    TotaldashLine.Pen = new Pen(Color.Green,0.5f);
    TotaldashLine.Pen.DashPattern = pattern;       
    plot.Add(TotaldashLine, NPlot.PlotSurface2D.XAxisPosition.Bottom, NPlot.PlotSurface2D.YAxisPosition.Right, 100);
    Grid oGrid = new Grid();
    oGrid.HorizontalGridType = Grid.GridType.Coarse;
    oGrid.VerticalGridType = Grid.GridType.Coarse;
    plot.Add(oGrid);       
     plot.Refresh();
    return plot;

}

TotaldashLine.Pen.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;