group (group, name) {
        if (!arguments.length) {
            return this._group;
        }
        this._group = group;
        this._groupName = name;
        this.expireCache();
        return this;
    }
**mandatory**

Set or get the group attribute of a chart. In `dc` a group is a
{@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#group-map-reduce crossfilter group}.
Usually the group should be created from the particular dimension associated with the same chart. If a value is
given, then it will be used as the new group.

If no value specified then the current group will be returned.
If `name` is specified then it will be used to generate legend label.
@see {@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#group-map-reduce crossfilter.group}
@example
var index = crossfilter([]);
var dimension = index.dimension(pluck('key'));
chart.dimension(dimension);
chart.group(dimension.group().reduceSum());
@param {crossfilter.group} [group]
@param {String} [name]
@returns {crossfilter.group|BaseMixin}