複数の凡例をプロットに追加できます

ScottPlot.Plot myPlot = new();

for (int i = 1; i <= 5; i++)
{
    double[] data = Generate.Sin(51, phase: .02 * i);
    var sig = myPlot.Add.Signal(data);
    sig.LegendText = $"Signal #{i}";
    sig.LineWidth = 2;
}

// デフォルトの凡例
var leg1 = myPlot.ShowLegend();
leg1.Alignment = Alignment.LowerRight;
leg1.Orientation = Orientation.Vertical;

// 追加の凡例
var leg2 = myPlot.Add.Legend();
leg2.Alignment = Alignment.UpperCenter;
leg2.Orientation = Orientation.Horizontal;

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