Hello shrndz,
at the moment it is possible to built a stacked area chart using one category and two series. If you have used only a serie and a second category to create the serie splitting you can do only a simple stacked bar and not a stackd area. I will report the issue to the development team and in the next days we will try to underestand if it is a bug or not.
Thank you for the warning. I hope you find this information useful, but if you were looking for something different, please answer again!