BarSeries.drawLabels() doesn't account for multiple yFields, throws NPEs
I have a BarSeries with 2 yFields and a labelConfig. My store has 9 items, so with two yFields that means there are 18 rects drawn. BarSeries.calculatePaths() has logic that alternates between the two fields for each of the 9 items. BarSeries.drawLabels(), however, does not -- it uses the number of rects to try to get the labels, so it's inaccurate and throws an exception once you exceed the store's bounds.
You should be able to reproduce this by adding a labelConfig to your grouped barchart example.