d3.zoom()

Creates a new zoom behavior. The returned behavior, zoom, is both an object and a function, and is typically applied to selected elements via selection.call.

返回一個既是對象又是函數，通過select.call()調用

zoom(selection)

Applies this zoom behavior to the specified selection, binding the necessary event listeners to allow panning and zooming, and initializing the zoom transform on each selected element to the identity transform if not already defined. This function is typically not invoked directly, and is instead invoked via selection.call. For example, to instantiate a zoom behavior and apply it to a selection:

就是一個調用函數
可以這樣調用

selection.call(d3.zoom().on("zoom", zoomed));