I found this when searching if there was a way to sort a stack chart - in my case I want it to be sorted by quantity, largest to smallest (in the legend as well as the chart) instead of by the default sheet order. The weird thing I just discovered is that if I create it as a bar chart, sort in the order I want and then change it to a stack chart - my sort order is retained! Same for pie and donut charts. Might be a decent workaround for now.