@MrNorth35 I think the interpolation was done in "screen coordinates" and I don't think the interpolated points are public (I have not checked this with the code). That probably means you need to derive a new LineSeries class. I think you need to transform the X to "screen X", then find the corresponding interpolated "screen Y", then inverse transform to get the Y.
@StehlikC Full intervals are not yet supported, and I don't think this feature is added to the issues yet. Can you add it?