In a chart that has stacked bar series and a line series the bar series become column series when the chart is data-bound.

The solution is to declare the BarSeries above the LineSeries as follows:
            <%--BarSeries are declared above the LineSeries--%>
            <telerik:BarSeries Name="B1">
                <Items>
                    <telerik:SeriesItem YValue="10" />
                    <telerik:SeriesItem YValue="20" />
                    <telerik:SeriesItem YValue="30" />
                </Items>
            </telerik:BarSeries>
            <%--LineSeries are declared below the BarSeries--%>
            <telerik:LineSeries Name="L1">
                <Items>
                    <telerik:SeriesItem YValue="15" />
                    <telerik:SeriesItem YValue="10" />
                    <telerik:SeriesItem YValue="20" />
                </Items>
            </telerik:LineSeries>