Make the drag behavior independent of the target element’s data?

[mbostock]

What if you wanted to use d3-drag on Canvas to drag circles? In that case, you wouldn’t be able to listen for mousedown on a specific element, since it would always be dragging on the Canvas element. But maybe there’s still a way to do it: the drag behavior could track the coordinates relative to the first click (and maybe there’s an equivalent to drag.origin in D3 3.x that lets you offset the coordinates to something more natural). You’d also presumably need something like drag.filter that lets you test whether you clicked on something before initiating the drag. And, when a drag event is dispatched, to know that’s being dragged (since there could be multiple things in motion).

[mbostock]

We’ll also need to observe multiple changedTouches during a touchstart.