SankeyLayoutAlgorithmBase Class

private void Form1_Load(object sender, EventArgs e) {
     SankeyDiagramControl sankeyDiagramControl1 = new SankeyDiagramControl();
     this.Controls.Add(sankeyDiagramControl1);
    List<SankeyItem> data = new List<SankeyItem> {
        new SankeyItem { Source = "France", Target = "UK", Value = 53 },
        new SankeyItem { Source = "Australia", Target = "UK", Value = 72 },
        new SankeyItem { Source = "France", Target = "Canada", Value = 81 },
        new SankeyItem { Source = "China", Target = "Canada", Value = 96 },
        new SankeyItem { Source = "UK", Target = "France", Value = 61 },
        new SankeyItem { Source = "Canada", Target = "France", Value = 89 } 
    };
    sankeyDiagramControl1.DataSource = data;
    sankeyDiagramControl1.SourceDataMember = "Source";
    sankeyDiagramControl1.TargetDataMember = "Target";
    sankeyDiagramControl1.WeightDataMember = "Value";

    sankeyDiagramControl1.LayoutAlgorithm = new MyLayoutAlgorithm();
    sankeyDiagramControl1.Width = 640;
    sankeyDiagramControl1.Height = 360;
}

public class SankeyItem {
    public string Source { get; set; }
    public string Target { get; set; }
    public double Value { get; set; }
}
public class MyLayoutAlgorithm : SankeyLayoutAlgorithmBase {

    void SpreadLevelIndex(ISankeyNodeLayoutItem node, int startingLevelIndex = 0) {
        node.LevelIndex = startingLevelIndex;
        if (node.OutputLinks == null)
            return;
        foreach (var outputLink in node.OutputLinks)
            SpreadLevelIndex(outputLink.Target, startingLevelIndex + 1);
    }
    public override void CalculateNodeBounds(IEnumerable<ISankeyNodeLayoutItem> nodes, Rectangle bounds) {
        foreach (var node in nodes)
            if (node.InputLinks == null || node.InputLinks.Count == 0)
                SpreadLevelIndex(node);
        int nodeWidth = bounds.Width / 10;
        int nodeHeight = bounds.Height / 10;
        int levelCount = nodes.Max(node => node.LevelIndex) + 1;
        int spaceBetweenLevels = (bounds.Width - nodeWidth) / (levelCount - 1);
        int maxNodeCountInALevel = nodes.GroupBy(node => node.LevelIndex).Max(group => group.Count());
        int spaceBetweenNodes = (bounds.Height - nodeHeight) / (maxNodeCountInALevel - 1);
        Dictionary<int, int> levelNodeCountPairs = new Dictionary<int, int>();
        foreach (var node in nodes) {
            if (!levelNodeCountPairs.TryGetValue(node.LevelIndex, out int nodeCount))
                levelNodeCountPairs.Add(node.LevelIndex, 0);
            levelNodeCountPairs[node.LevelIndex]++;
            node.Bounds = new Rectangle(bounds.Left + node.LevelIndex * spaceBetweenLevels, bounds.Top + nodeCount * spaceBetweenNodes, nodeWidth, nodeHeight);
        }
    }
}

Private Sub Form1_Load(ByVal sender As Object, ByVal e As EventArgs)
    Dim sankeyDiagramControl1 As SankeyDiagramControl = New SankeyDiagramControl()
    Me.Controls.Add(sankeyDiagramControl1)
    Dim data As List(Of SankeyItem) = New List(Of SankeyItem) From {
        New SankeyItem With {
            .Source = "France",
            .Target = "UK",
            .Value = 53
        },
        New SankeyItem With {
            .Source = "Australia",
            .Target = "UK",
            .Value = 72
        },
        New SankeyItem With {
            .Source = "France",
            .Target = "Canada",
            .Value = 81
        },
        New SankeyItem With {
            .Source = "China",
            .Target = "Canada",
            .Value = 96
        },
        New SankeyItem With {
            .Source = "UK",
            .Target = "France",
            .Value = 61
        },
        New SankeyItem With {
            .Source = "Canada",
            .Target = "France",
            .Value = 89
        }
    }
    sankeyDiagramControl1.DataSource = data
    sankeyDiagramControl1.SourceDataMember = "Source"
    sankeyDiagramControl1.TargetDataMember = "Target"
    sankeyDiagramControl1.WeightDataMember = "Value"
    sankeyDiagramControl1.LayoutAlgorithm = New MyLayoutAlgorithm()
    sankeyDiagramControl1.Width = 640
    sankeyDiagramControl1.Height = 360
End Sub

Public Class SankeyItem
    Public Property Source As String
    Public Property Target As String
    Public Property Value As Double
End Class

Public Class MyLayoutAlgorithm
    Inherits SankeyLayoutAlgorithmBase

    Private Sub SpreadLevelIndex(ByVal node As ISankeyNodeLayoutItem, ByVal Optional startingLevelIndex As Integer = 0)
        node.LevelIndex = startingLevelIndex
        If node.OutputLinks Is Nothing Then Return

        For Each outputLink In node.OutputLinks
            SpreadLevelIndex(outputLink.Target, startingLevelIndex + 1)
        Next
    End Sub

    Public Overrides Sub CalculateNodeBounds(ByVal nodes As IEnumerable(Of ISankeyNodeLayoutItem), ByVal bounds As Rectangle)
        For Each node In nodes
            If node.InputLinks Is Nothing OrElse node.InputLinks.Count = 0 Then SpreadLevelIndex(node)
        Next

        Dim nodeWidth As Integer = bounds.Width / 10
        Dim nodeHeight As Integer = bounds.Height / 10
        Dim levelCount As Integer = nodes.Max(Function(node) node.LevelIndex) + 1
        Dim spaceBetweenLevels As Integer = (bounds.Width - nodeWidth) / (levelCount - 1)
        Dim maxNodeCountInALevel As Integer = nodes.GroupBy(Function(node) node.LevelIndex).Max(Function(group) group.Count())
        Dim spaceBetweenNodes As Integer = (bounds.Height - nodeHeight) / (maxNodeCountInALevel - 1)
        Dim levelNodeCountPairs As Dictionary(Of Integer, Integer) = New Dictionary(Of Integer, Integer)()
        Dim nodeCount As Integer = Nothing

        For Each node In nodes
            If Not levelNodeCountPairs.TryGetValue(node.LevelIndex, nodeCount) Then levelNodeCountPairs.Add(node.LevelIndex, 0)
            levelNodeCountPairs(node.LevelIndex) += 1
            node.Bounds = New Rectangle(bounds.Left + node.LevelIndex * spaceBetweenLevels, bounds.Top + nodeCount * spaceBetweenNodes, nodeWidth, nodeHeight)
        Next
    End Sub
End Class