zoom.scaleBy()

zoom.scaleBy(selection, k)

If selection is a selection, scales the current zoom transform of the selected elements by k, such that the new k₁ = k₀ × k. If selection is a transition, defines a “zoom” tween translating the current transform. This method is a convenience method for zoom.transform. The k scale factor may be specified either as numbers or as functions that returns numbers. If a function, it is invoked for each selected element, being passed the current datum d and index i, with the this context as the current DOM element.