function renderItem(params, api) {

    ... api.coord(param) ...

    ...

    return {
        type: 'group',
        children: [{
            type: 'line',
            ...
            style: style
        }, {
            type: 'line',
            ...,
            style: style
        }, {
            type: 'line',
            ...,
            style: style
        },
        ...
        ]
    };
}