outline for Area Series | Infragistics Forums

This should get you most of the way there. You can create a visual at the reference point by creating an extra crossing axis.
You can create more of an outline for the area series by using an implicit style for the paths that it uses. But one of the path figures is still not closed.
Because the chart does not intend for the figure to be outline in this way.
If you want to enable full outlining of the area series, I would make a feature request.
Here is the Xaml:

<Grid x:Name="LayoutRoot" Background="White">
        <igChart:XamDataChart x:Name="theChart">
            <igChart:XamDataChart.Axes>
                <igChart:NumericYAxis x:Name="yAxis" />
                <igChart:CategoryXAxis 
                    x:Name="xAxis"
                    ItemsSource="{StaticResource data}"
                    Label="{}{Label}"/>

                <igChart:CategoryXAxis x:Name="referenceAxis"
                                      Stroke="Red"
                                      StrokeThickness="3"
                                      ItemsSource="{StaticResource data}"
                                      CrossingAxis="{Binding ElementName=yAxis}"
                                      CrossingValue="0"
                                      >
                    <igChart:CategoryXAxis.LabelSettings>
                        <igChart:AxisLabelSettings 
                            Location="InsideBottom"
                            Visibility="Collapsed"/>
                    </igChart:CategoryXAxis.LabelSettings>
                </igChart:CategoryXAxis>
            </igChart:XamDataChart.Axes>
            <igChart:XamDataChart.Series>
                
                <igChart:ColumnSeries
                    x:Name="series2"
                    ItemsSource="{StaticResource data}"
                    XAxis="{Binding ElementName=xAxis}"
                    YAxis="{Binding ElementName=yAxis}"
                    ValueMemberPath="Value" />

                <igChart:AreaSeries
                    x:Name="series"
                    Thickness="3"
                    Outline="Gray"
                    ItemsSource="{StaticResource data}"
                    XAxis="{Binding ElementName=xAxis}"
                    YAxis="{Binding ElementName=yAxis}"
                    ValueMemberPath="Value" >
                    <igChart:AreaSeries.Resources>
                        <Style TargetType="Path">
                            <Setter Property="Stroke" Value="Gray" />
                            <Setter Property="StrokeThickness" Value="3" />
                        </Style>
                    </igChart:AreaSeries.Resources>
                </igChart:AreaSeries>
            </igChart:XamDataChart.Series>
        </igChart:XamDataChart>

Hope this helps!
-Graham