normalize (normalize) {
        if (!arguments.length) {
            return this._normalize;
        }
        this._normalize = normalize;
        return this;
    }
This function will be called on values before calling the filter function.
@example
// This is the default
chart.normalize(function (s) {
  return s.toLowerCase();
});
@param {function} [normalize]
@returns {TextFilterWidget|function}