function countSimply() {
    return this._data.length;
}
Compatible with some cases (in pie, map) like:
data: [{name: 'xx', value: 5, selected: true}, ...]
where only sourceFormat is 'original' and 'objectRows' supported.

??? TODO
Supported detail options in data item when using 'arrayRows'.

@param {module:echarts/data/List} data
@param {number} dataIndex
@param {string} attr like 'selected'