Since ShinobiChart doesn't have a spline series, you would need to use a line series instead and use a high number of points to make it appear like a curve.
Say you had 10 data values, you would need to work out the points you would need to plot to create a spline for them. You would want to calculate a high number of points on the spline, for example 100, and then return these 100 values to the chart. The chart would plot these lines and line drawn would be close to the spline you are after. If you wanted a more accurate spline, you would calculate more than 100 points on your spine.
We don't have a demo of this being done, however, giving the points to your chart is demonstrated in every one of our chart examples. How to calculate points along a spline is more of a mathematical question.