Chart for .NET > User's Guide > Chart Types > Smooth Line > Standard_Smooth_Line 
Smooth Line charts are similar to Line charts, the difference being that the successive data points are connected with cubic spline segments instead of straight lines. The following images display Smooth Line charts.
Figure 1.
Smooth Line series are represented by the NSmoothLineSeries type. An instance of this type must be added to the series collection of a Cartesian chart.
C# 
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// obtain a reference to the Cartesian chart that is created by default NCartesianChart chart = (NCartesianChart)chartControl.Charts[0]; // add a smooth line series to it NSmoothLineSeries smoothline = (NSmoothLineSeries)chart.Series.Add(SeriesType.SmoothLine); 
Visual Basic 
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' obtain a reference to the Cartesian chart that is created by default Dim chart As NCartesianChart = chartControl.Charts(0) ' add a smooth line series to it Dim smoothline As NSmoothLineSeries = chart.Series.Add(SeriesType.SmoothLine) 
It is often required to place the data points at custom positions along the x axis. If you set the UseXValues property to true the smooth line will use the custom x coordinates specified in the XValues data series. In this case you are able to select the spline interpolation method with the help of the Use1DInterpolationForXYScatter property. If this property is set to true, the elevation values are regarded as a function of the x values. This means that there is always only one Y for every X coordinate. When the property is set to false, the line is interpolated as a free 2D line in the XY plane. The following figures show two curves produced from the same data, but with different interpolation methods.
Demonstration of different interpolation modes  

1D interpolation  2D interpolation  