data (callback) {
        if (!arguments.length) {
            return this._data(this._group);
        }
        this._data = typeof callback === 'function' ? callback : utils.constant(callback);
        this.expireCache();
        return this;
    }
Set the data callback or retrieve the chart's data set. The data callback is passed the chart's
group and by default will return
{@link https://github.com/crossfilter/crossfilter/wiki/API-Reference#group_all group.all}.
This behavior may be modified to, for instance, return only the top 5 groups.
@example
// Default data function
chart.data(function (group) { return group.all(); });

chart.data(function (group) { return group.top(5); });
@param {Function} [callback]
@returns {*|BaseMixin}