I don't know whether this kind of chart is possible but perhaps someone can help me.
I want to create a stacked chart with groups e.g. 6 sersies divided into two stacked bars (there are 2 bars per label, each stacked of three values)
Does anyone know how can I do it?
You are asking for a Combination Chart. By default, it's not possible in APEX.
However it looks like you can do it using anychart (charting engine behind apex charts). See Chart Combinations