Customizing zoomRect via zoomFunctions

[joshkel]

Normal zooming supports customizing zoom behavior using zoomFunctions, but the drag-to-zoom functionality that's implemented by zoomRect bypasses zoomFunctions and calls updateRange correctly.

I have a custom scale implementation that relies on zoomFunctions, so trying to enable drag-to-zoom causes problems. (Specifically, I'm breaking up the axis into viewports so that I can stack multiple series within this chart - somewhat like this example. The scales for the individual portions of the chart shouldn't be directly zoomed, because their layout is managed by a separate, controlling scale, so their zoomFunctions are no-ops.)

Zooming by rectangle appears to be a distinctly different interface than zooming by percentage and focal point, so would it be reasonable to add a zoomRectFunctions to let scales customize that behavior and cover this use case? Or is this too niche/specialized?

And, if it's worth adding zoomRectFunctions, would it be worth exposing zoomRect as part of the public API, like zoom and zoomScale?