Please excuse us for the delayed response.
You can override the default resizing logic of the RadDiagram by overriding/extending its ResizingService. You can find information about the Services of the RadDiagram in this help article
. By inheriting the default ResizingService and overriding its Resize method you can manipulate the new point for resizing and therefor keep the Ratio of an shape based on Attached property or simply keep the ratio of all shapes. I have prepared a sample project demonstrating the approach, please keep in mind that there might be some border cases that need to be polished.
As for your other question you about keeping the rounded corners ratio. The Shape is displaying its Geometry in a Path which Stretch property is set to Fill which makes the path to fill the entire Shape based on its size. The RadDiagram is not manipulating the Geometry when you resize the shape and that is why the rounding is lost. You could use the SizeChanged event of the RadDiagramShape and recalculate its geometry based on some custom logic. However such recalculation will be though to calculate.
I hope this information helps.
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