Hi
Hmmm... this might be getting to the point where I'm trying to punch above my weight 😉
You can get Power BI to provide a higher-level dataView if you've enabled drilling on the specified dataRole, but this is via the conventional buttons on the visual header, and/or through the context menu, if you've enabled it. I have this working very nicely on a visual using the categorical data view mapping. Doing this programmatically with elements in the visual is where I might get stuck.
If I were to approach the challenge of a matrix with expandable/collapsible groups, I'd start with the approach of ensuring that my dataView contained all the data I needed for all levels and would then map my view model to show/hide each node/level accordingly, and providing a toggle control in the approrpiate state to expand/collapse as appropriate. This comes with the disclaimer that I haven't really thought about this before today...
There's not so many people on the forums who chime in on custom visuals development, so I'm not sure (but hoping) that we might get something more timely for you from someone else. Have you tried asking the custom visuals team about this one? The can be contacted at pbicvsupport@microsoft.com if you haven't tried them. They do frequent the Custom Visuals Development Discussion forum on this board as well, but this might be a specific enough challenge that a direct question could be warranted.
Regards,
Daniel
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