I am a bit confused. Do you still need our help?
You mentioned that the observable collection is set in some events. If these events are in your view models, you can execute a quick check when they occur - if the observable collection is empty or not. If the collection is empty - the chart is empty. So you can expose a new property in your view models to indicate whether or not there is any data.
Even if these events are not in the view models, you can add a collection changed handler to the observable collection and get the same results.
Once you introduce this IsDataEmpty property in your view models, you can easily update the UI when there is and when there is no data.
Let us know if you need further assistance.
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