Hi Zabrina,
the method you are searching for is:
DS_1.sortByMeasure("4BSN2BA7P20Z3RJI9JZI052Y9", true);
but, this will sort the data source, so if you have crosstab abd chart on the same data source, both will change directions.
If you want to have an oposite sort order, the only way for today I can see is to have 2 data sources for this.