Coordinate[] polygon =
{
    new Coordinate(30, 15),
    new Coordinate(55, 40),
    new Coordinate(60, 45),
    new Coordinate(80, 60),
    new Coordinate(40, 95),
    new Coordinate(15, 90),
    new Coordinate(5, 50),
};

double[,] data = ScottPlot.DataGen.SampleImageData();
var heatmap = formsPlot1.Plot.AddHeatmap(data);
heatmap.ClippingPoints = polygon;