棒のコレクションは、単一の項目として凡例に表示できます。

ScottPlot.Plot myPlot = new();

double[] xs1 = { 1, 2, 3, 4 };
double[] ys1 = { 5, 10, 7, 13 };
var bars1 = myPlot.Add.Bars(xs1, ys1);
bars1.LegendText = "Alpha";

double[] xs2 = { 6, 7, 8, 9 };
double[] ys2 = { 7, 12, 9, 15 };
var bars2 = myPlot.Add.Bars(xs2, ys2);
bars2.LegendText = "Beta";

myPlot.ShowLegend(Alignment.UpperLeft);
myPlot.Axes.Margins(bottom: 0);

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