I am trying to databind the expanded property of the org chart nodes.  The closest I could come was to set the IsChecked property on a togglebutton style for the expander.

<Style x:Key="ExpansionIndicatorStyle" TargetType="ToggleButton">
            <Setter Property="IsChecked" Value="{Binding IsExpanded, Mode=TwoWay}"/>
</Style>
...
<ig:OrgChartNodeLayout TargetTypeName="AllocationNode"
                                       DisplayMemberPath="Name"
                                       Key="Children"
                                       ExpansionIndicatorStyle="{StaticResource ExpansionIndicatorStyle}"/>


However, the state of IsChecked seems to be ignored by the layout.  What am I doing wrong?