等高線はヒートマップの上に配置できます。

ScottPlot.Plot myPlot = new();

Coordinates3d[,] cs = new Coordinates3d[50, 50];
for (int y = 0; y < cs.GetLength(0); y++)
{
    for (int x = 0; x < cs.GetLength(1); x++)
    {
        double z = Math.Sin(x *.1) + Math.Cos(y* .1);
        cs[y, x] = new(x, y, z);
    }
}

var heatmap = myPlot.Add.Heatmap(cs);
heatmap.FlipVertically = true;
heatmap.Colormap = new ScottPlot.Colormaps.MellowRainbow();

var contour = myPlot.Add.ContourLines(cs);
contour.LabelStyle.Bold = true;
contour.LinePattern = LinePattern.DenselyDashed;
contour.LineColor = Colors.Black.WithAlpha(.5);

myPlot.Axes.TightMargins();
myPlot.HideGrid();

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