D3 zoom

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.

返回一个既是对象又是函数，通过select.call()调用

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:

就是一个调用函数
可以这样调用

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