htmlSection (htmlSection) {
        if (!arguments.length) {
            return this._htmlSection;
        }
        this._htmlSection = htmlSection;
        return this;
    }
Get or set the function that formats a section label.
@example
chart.htmlSection (function (d) { return '<h2>'.d.key . 'with ' . d.values.length .' items</h2>'});
@param {Function} [htmlSection]
@returns {Function|DataGrid}