Contour Plots

Install: dotnet add package MatPlotLibNet

Contour lines with labels

double[] x = Enumerable.Range(0, 20).Select(i => i * 0.5 - 5.0).ToArray();
double[] y = Enumerable.Range(0, 20).Select(i => i * 0.5 - 5.0).ToArray();
var z = new double[20, 20];
for (int r = 0; r < 20; r++)
    for (int c = 0; c < 20; c++)
        z[r, c] = Math.Sin(x[c]) * Math.Cos(y[r]);

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .WithTitle("Contour with Labels")
        .Contour(x, y, z, s =>
        {
            s.ShowLabels = true;
            s.LabelFormat = "F2";
            s.LabelFontSize = 9;
        })
        .WithColorMap("coolwarm"))
    .Save("contour_labels.svg");

Contour with labels

A colour map name is checked. AxesBuilder.WithColorMap(string) throws ArgumentException when the name is unknown, for example the typo "virdis", and the exception lists the registered names, so a typo never falls back to a default map in silence. If you want a lookup that does not throw, fetch the instance yourself with ColorMapRegistry.Get(name) and call the WithColorMap(IColorMap) overload only when the result is not null.

Filled contour (contourf)

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .WithTitle("Filled Contour")
        .Contourf(x, y, z, s =>
        {
            s.Levels = 15;
            s.ColorMap = ColorMaps.Viridis;
        })
        .WithColorBar(cb => cb with { Label = "Amplitude" }))
    .Save("contourf.svg");

Custom contour levels

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .Contour(x, y, z, s =>
        {
            // Explicit level values instead of auto count
            s.LevelValues = [-0.8, -0.4, 0, 0.4, 0.8];
            s.ShowLabels = true;
            s.LabelFormat = "F1";
        })
        .WithColorMap("RdBu"))
    .Save("contour_custom_levels.svg");

The levels you name are the levels drawn, ascending and each once; Levels is ignored while LevelValues holds a value. A level outside the range of the grid crosses no cell, so it draws no line, and a level that is not a number is left out. A filled contour (Contourf) takes LevelValues too: it fills one band between each pair of neighbouring levels and runs its colour map and colour bar from the first level to the last, whatever range the grid spans. Its lowest band is painted under the whole plot, so where the grid lies below the first level you name, the plot shows that band's colour; matplotlib leaves that area empty unless asked to extend the colours.

Contour with level count

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .Contour(x, y, z, s =>
        {
            s.Levels = 20;           // 20 contour lines
            s.ShowLabels = false;    // no inline labels
        })
        .WithColorMap("plasma")
        .WithColorBar())
    .Save("contour_dense.svg");

Contour + filled contour overlay

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .WithTitle("Filled + Line Contour")
        .Contourf(x, y, z, s => { s.Levels = 12; s.ColorMap = ColorMaps.Coolwarm; })
        .Contour(x, y, z, s =>
        {
            s.Levels = 12;
            s.ShowLabels = true;
            s.LabelFormat = "F2";
        })
        .WithColorBar())
    .Save("contour_overlay.svg");

Triangulated contour

Use a triangulated contour for irregularly spaced data. The points are connected by Delaunay triangulation:

Plt.Create()
    .AddSubPlot(1, 1, 1, ax => ax
        .Tricontour(xIrregular, yIrregular, zIrregular)
        .WithColorMap("viridis")
        .WithColorBar())
    .Save("tricontour.svg");

Fluent API reference — ContourSeries

Property	Type	Default	Description
`Levels`	`int`	`10`	Number of contour levels
`LevelValues`	`double[]`	none	The levels to draw, ascending, each once; `Levels` is ignored when set
`Filled`	`bool`	`false`	Filled areas (contourf) instead of lines only
`ShowLabels`	`bool`	`false`	Show level values on contour lines
`LabelFormat`	`string`	`"G4"`	.NET format string for labels
`LabelFontSize`	`double`	`10`	Label font size
`ColorMap`	`IColorMap`	auto	Colormap for level colors

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