FillY プロットは、X、Y1、Y2 配列から作成できます。

ScottPlot.Plot myPlot = new();

RandomDataGenerator dataGen = new(0);

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

var fill = myPlot.Add.FillY(xs, ys1, ys2);
fill.FillColor = Colors.Blue.WithAlpha(100);
fill.LineColor = Colors.Blue;
fill.MarkerColor = Colors.Blue;
fill.LineWidth = 2;

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