If you double-click the Group by option for your Flashboards variable, your Flashboard will display data in multiple pie charts instead of one pie chart.
For example, if your Flashboard is a pie chart and you grouped a variable that retrieves tickets by Assignee, and then by Priority, your resulting Flashboard display will consist of several pie charts, each of which corresponds to one assignee.
If you want to display a Flashboard with a double Group By option, use another type of flashboard.