public string WeightDataMember { get; set; }

Public Property WeightDataMember As String

<Window
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:local="clr-namespace:SankeySample"
        xmlns:dxsa="http://schemas.devexpress.com/winfx/2008/xaml/sankey" 
        x:Class="SankeySample.MainWindow"
        mc:Ignorable="d"
        Title="MainWindow" Height="450" Width="800">
    <Grid>
        <dxsa:SankeyDiagramControl DataSource="{Binding Data}" 
                                   SourceDataMember="Source" 
                                   TargetDataMember="Target" 
                                   WeightDataMember="Value">
            <dxsa:SankeyDiagramControl.DataContext>
                <local:SankeyViewModel/>
            </dxsa:SankeyDiagramControl.DataContext>
        </dxsa:SankeyDiagramControl>
    </Grid>
</Window>

using System.Collections.Generic;
using System.Windows;

namespace SankeySample {
//...
    public class SankeyViewModel {
        public List<SankeyItem> Data {
            get { return GetData(); }
        }
        public List<SankeyItem> GetData() {
            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 } 
            };
            return data;
        }
    }
    public class SankeyItem {
        public string Source { get; set; }
        public string Target { get; set; }
        public double Value { get; set; }
    }
}

Imports System.Collections.Generic

Namespace SankeySample
    '...
    Public Class SankeyViewModel
        Public ReadOnly Property Data As List(Of SankeyItem)
            Get
                Return GetData()
            End Get
        End Property

        Public Function GetData() As List(Of SankeyItem)
            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
                }
            }
            Return data
        End Function
    End Class

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