Separate positive and negative fill colors for areas (for difference charts), or an area clipping channel or mark (and then use two marks).

[mbostock]

[Fil]

differenceArea plugin in https://observablehq.com/@observablehq/plot-difference-chart

[Fil]

My plugin mentioned above is infinitely more complex than filling the space with vertical rules, as Zan suggested.

[mbostock]

It looks like since June 29 the difference chart example no longer uses clipping? But I believe that approach isn’t viable when the curve is affected by multiple points (even in the case of linear, but most acutely with basis)?

Here is a take using a render transform and a compound mark:

https://observablehq.com/d/1e37e3c09876e460

Before:

After:

[Fil]

Yes I simplified it since in that case the clipping approach was overkill, and I wanted to revisit the implementation when we did #1109 (Also, you may have noticed that I had inadvertently switched the colors!)