I'm using Google Charts API 1.1 but i simply can't make grouped stacks. I've used the "targetAxisIndex" separating them into two axis and it kinda works but if i don't set max value for each yAxis they are scaled differently and it's useless once both of the grouped stacks actually refers to the same thing.
I'd like to know i can make something similar to this: http://www.highcharts.com/demo/column-stacked-and-grouped