The behavior you describe is expected, because the spline curve is actually an approximation. Basically, the curve runs through the provided control points - the actual DataPoints. When the path between any two of these points is calculated, it takes into account their adjacent "outer" points. If this does not meet your requirements you can use the line type
instead. Have a look at the following online resources for an overview of the Line Charts
I hope this helps.
the Telerik team