Since this question was originally asked almost 4 years ago, the answer was not as easy as it is now, however, the latest version of the Chart widget in Mendix 10.6 has a 'stacked' option. A similar option exists for the column version of the chart, which will be called Column format.
Since the underlying library had support for this 4 years ago as well, it would likely have sufficed to add the appropriate settings in the Advanced tab, something I suspect the author of this question did in the end.
Koen van Aken, next time you can also post your own question, which would allow for more discussion.