stack (group, name, accessor) {
        if (!arguments.length) {
            return this._stack;
        }

        if (arguments.length <= 2) {
            accessor = name;
        }

        const layer = {group: group};
        if (typeof name === 'string') {
            layer.name = name;
        }
        if (typeof accessor === 'function') {
            layer.accessor = accessor;
        }
        this._stack.push(layer);

        return this;
    }
Stack a new crossfilter group onto this chart with an optional custom value accessor. All stacks
in the same chart will share the same key accessor and therefore the same set of keys.

For example, in a stacked bar chart, the bars of each stack will be positioned using the same set
of keys on the x axis, while stacked vertically. If name is specified then it will be used to
generate the legend label.
@see {@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#group-map-reduce crossfilter.group}
@example
// stack group using default accessor
chart.stack(valueSumGroup)
// stack group using custom accessor
.stack(avgByDayGroup, function(d){return d.value.avgByDay;});
@param {crossfilter.group} group
@param {String} [name]
@param {Function} [accessor]
@returns {Array<{group: crossfilter.group, name: String, accessor: Function}>|StackMixin}