The SplineArea chart is just like the area chart with the only difference in the manner in which data points are connected.  The SplineArea chart connects data points by using splines instead of straight lines, and fills the area enclosed by the splines.

Set the ChartType property to SplineArea in the Properties window, in the code behind, or in XAML to create the SplineArea Chart.

To create the stacking WPF SplineArea Chart, set the Stacking property to Stacked or Stacked100pc.

WPF SplineArea Chart

Here is the implementation in code:

<c1:C1FlexChart x:Name="flexChart" 
                BindingX="Country" 
                ItemsSource="{Binding DataContext.Data}" 
                ChartType="SplineArea">
    <c1:C1FlexChart.Series>
        <c1:Series SeriesName="Sales" 
                   Binding="Sales"/>
        <c1:Series SeriesName="Expenses" 
                   Binding="Expenses"/>
    </c1:C1FlexChart.Series>
</c1:C1FlexChart>

flexChart.ChartType = C1.Chart.ChartType.SplineArea;