function renderItemHexBin(params, api) {
    var center = api.coord([api.value(0), api.value(1)]);
    var points = ...
    ...

    return {
        type: 'group',
        children: [{
            type: 'polygon',
            shape: {
                points: points
            },
            style: {
                stroke: '#ccc',
                fill: api.visual('color'),
                lineWidth: 0
            }
        },
        ...
        ]
    };
}