multiple (multiple) {
        if (!arguments.length) {
            return this._multiple;
        }
        this._multiple = multiple;
        if (this._multiple) {
            this._inputType = 'checkbox';
        } else {
            this._inputType = 'radio';
        }
        return this;
    }
Controls the type of input element. Setting it to true converts
the HTML `input` tags from radio buttons to checkboxes.
@param {boolean} [multiple=false]
@returns {Boolean|CboxMenu}
@example
chart.multiple(true);