brush.extent([extent])
If extent is specified, sets the brushable extent to the specified array of points [[x0, y0], [x1, y1]], where [x0, y0] is the top-left corner and [x1, y1] is the bottom-right corner, and returns this brush. The extent may also be specified as a function which returns such an array; 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. If extent is not specified, returns t