[NonCategorized]
public ElementSelectionMode SelectionMode { get; set; }

<NonCategorized>
Public Property SelectionMode As ElementSelectionMode

<dxh:HeatmapControl x:Name="heatmap" 
                    Grid.Row="0" Margin="10" 
                    SelectionMode="Multiple"
                    SelectedItems="{Binding SelectedSales, Mode=TwoWay}" 
                    ToolTipEnabled="True">
    <dxh:HeatmapDataSourceAdapter
        DataSource="{Binding Sales}"
        XArgumentDataMember="Month"
        YArgumentDataMember="Product"
        XArgumentComparer="{local:SaveOrderComparer}"
        ColorDataMember="RevenueByMonth"/>
        <!--...-->
</dxh:HeatmapControl>

<dxc:ChartControl x:Name="chart" Grid.Row="1"
    DataSource="{Binding SelectedSales, Converter={local:SelectedItemsToDataSourceConverter}}" 
    BorderBrush="Transparent">
    <!--...-->
</dxc:ChartControl>

public class InteractionViewModel : BindableBase {
    //...

    readonly List<ProductSale> sales;
    ObservableCollection<object> selectedSales;

    public List<ProductSale> Sales { get { return sales; } }
    public virtual ObservableCollection<object> SelectedSales {
        get { return selectedSales; }
        set {
            if (selectedSales == value)
                return;
            selectedSales = value;
            if (selectedSales != null)
                selectedSales.CollectionChanged += (s, e) => RaisePropertyChanged("SelectedSales");
            RaisePropertyChanged("SelectedSales");
        }
    }
    protected InteractionViewModel() {
        sales = LoadData();
        SelectedSales = new ObservableCollection<object>() { sales[0] };
    }
    //...
}

Public Class InteractionViewModel
        Inherits DevExpress.Mvvm.BindableBase
        '...
        Private ReadOnly salesField As System.Collections.Generic.List(Of TreeMapDemo.ProductSale)

        Private selectedSalesField As System.Collections.ObjectModel.ObservableCollection(Of Object)

        Public ReadOnly Property Sales As List(Of TreeMapDemo.ProductSale)
            Get
                Return Me.salesField
            End Get
        End Property

        Public Overridable Property SelectedSales As ObservableCollection(Of Object)
            Get
                Return Me.selectedSalesField
            End Get

            Set(ByVal value As ObservableCollection(Of Object))
                If Me.selectedSalesField Is value Then Return
                Me.selectedSalesField = value
                If Me.selectedSalesField IsNot Nothing Then AddHandler Me.selectedSalesField.CollectionChanged, Sub(s, e) Me.RaisePropertyChanged("SelectedSales")
                Me.RaisePropertyChanged("SelectedSales")
            End Set
        End Property

        Protected Sub New()
            Me.salesField = Me.LoadData()
            Me.SelectedSales = New System.Collections.ObjectModel.ObservableCollection(Of Object)() From {Me.salesField(0)}
        End Sub
        ' ...
    End Class