Possible to restrict trendline to non-negative values?

[RoxKilly]

I have a line chart through only a dozen points. Take a look at the span between 4 and 5:

The values represented can never be negative, so I'd like to prevent the line from going below zero. On the other hand I don't want to use straight lines to link the points. So my question is: does anyone know how to restrict curved trendline to non-negative values?

[etimberg]

Duplicate of #2768

I'd be happy to merge a PR that reverses #2758 but adds an option to turn it off