Plot Groups of Stacked Bars

Generate a bar plot with multiple stacks grouped together.

Plot a set of stacked bars, but group them according to labels provided.

Params:
stackData is a 3D matrix (i.e., stackData(i, j, k) => (Group, Stack, StackElement))
groupLabels is a CELL type (i.e., { 'a', 1 , 20, 'foo' };)