Thanks for the fast answer! That’s an interesting trick, but a bit clumsy, and the results are unusable in most situations: look at the gaps when the points around the pos/neg flips are not close to zero:
Beats the purpose of a spline. I have many situations where just 1 or 2 data points are below zero (and the x axis has dates), so I cannot just invent some x values with y=0 around those points.
I think a commercial product like CanvasJS should have a built-in solution for a trivial use case like this.