stackLayout (_stack) {
        if (!arguments.length) {
            return this._stackLayout;
        }
        this._stackLayout = _stack;
        return this;
    }
Gets or sets the stack layout algorithm, which computes a baseline for each stack and
propagates it to the next.
@see {@link https://github.com/d3/d3-3.x-api-reference/blob/master/Stack-Layout.md d3.stackD3v3}
@param {Function} [_stack=d3.stackD3v3]
@returns {Function|StackMixin}