FillY プロットは、public プロパティを使用してカスタマイズできます。

ScottPlot.Plot myPlot = new();

int count = 20;
double[] xs = Generate.Consecutive(count);
double[] ys1 = Generate.RandomWalk(count, offset: -5);
double[] ys2 = Generate.RandomWalk(count, offset: 5);

var fill = myPlot.Add.FillY(xs, ys1, ys2);
fill.MarkerShape = MarkerShape.FilledDiamond;
fill.MarkerSize = 15;
fill.MarkerColor = Colors.Blue;
fill.LineColor = Colors.Blue;
fill.LinePattern = LinePattern.Dotted;
fill.LineWidth = 2;
fill.FillColor = Colors.Blue.WithAlpha(.2);
fill.FillHatch = new ScottPlot.Hatches.Striped(ScottPlot.Hatches.StripeDirection.DiagonalUp);
fill.FillHatchColor = Colors.Blue.WithAlpha(.4);
fill.LegendText = "塗りつぶし領域";

myPlot.ShowLegend();

myPlot.SavePng("demo.png", 400, 300);