軸スパンは広範囲にカスタマイズできます。

ScottPlot.Plot myPlot = new();

myPlot.Add.Signal(Generate.Sin());
myPlot.Add.Signal(Generate.Cos());

var hs = myPlot.Add.HorizontalSpan(10, 20);
hs.LegendText = "こんにちは";
hs.LineStyle.Width = 2;
hs.LineStyle.Color = Colors.Magenta;
hs.LineStyle.Pattern = LinePattern.Dashed;
hs.FillStyle.Color = Colors.Magenta.WithAlpha(.2);

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