From the snapshot I can see that you are using the old RadChart. We have two charting solutions - old RadChart and new RadChartView. The new chartview is actually a set of controls - RadCartesianChart, RadPolarChart, RadPieChart and ChartDataSource.
Naturally, we suggest you use the new chart and remove any references to the old Charting.dll. The new control is much faster, very flexible and easy to set up. With the chartview, you need to set the CombineMode property of the series in order to get the desired results.
(you may want to install the sdk samples browser