Method group

src/base/stack-mixin.js:124–135

(g, n, f)

    }

    group (g, n, f) {
        if (!arguments.length) {
            return super.group();
        }
        this._stack = [];
        this._titles = {};
        this.stack(g, n);
        if (f) {
            this.valueAccessor(f);
        }
        return super.group(g, n);
    }

    /**
     * Allow named stacks to be hidden or shown by clicking on legend items.