brush (_) {
        if (!arguments.length) {
            return this._brush;
        }
        this._brush = _;
        return this;
    }
Get or set the brush. Brush must be an instance of d3 brushes
https://github.com/d3/d3-brush/blob/master/README.md
You will use this only if you are writing a new chart type that supports brushing.

**Caution**: dc creates and manages brushes internally. Go through and understand the source code
if you want to pass a new brush object. Even if you are only using the getter,
the brush object may not behave the way you expect.

@param {d3.brush} [_]
@returns {d3.brush|CoordinateGridMixin}