Column charts do not stack without stack_group if yaxis is set

[scop]

Checklist

• I updated the card to the latest version available
• I cleared the cache of my browser
• I verified that I'm really running the lastest version in my browser console
• I checked if there is another issue opened with the same problem

Describe the bug

Since version 2.1.0, a top level stacked: true is not enough to cause column series to stack if a top level yaxis configuration is present.

If no top level yaxis is present, stacking works.

The workaround is to assign series manually to stack_groups, but I feel it's a bug or at least weird behavior that it needs to be done only if an yaxis config is present.

Version of the card
Version: 2.1.2

To Reproduce
This is the configuration I used:

Screenshots
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Expected behavior
A clear and concise description of what you expected to happen.

Desktop (please complete the following information):

• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context

See debugging details at https://community.home-assistant.io/t/apexcharts-card-stacked-bar-graph-doesnt-stack-when-yaxis-is-included/810829

[scop]

Found #730 after filing this, seems related, if not duplicate.