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It's become much more than just an analytic tool by now, containing also
schema and survey management.

QtCharts can't deal with charts being deleted, so we need to update them
rather than to re-create them.

The singular chart now also shows correct values.

This useful with QtCharts using hardcoded roles.

This avoids having to special-case it all over the place.

We now have a dedicated class per aggregation handling model setup and
chart creation.