I created a simple project and I was able to reproduce the issue at hand. It appears that the chart has some internal logic for setting the EmptyContent property in case the EmptyContent is not set. This code is executed before the binding kicks in and since its set through the property setter, the value from the binding is not taken into consideration.
I am unsure if this is a flaw in the control or by design. Anyway, you can easily work this out with an attached property. The value of the attached property will be bound to the view model's property and in the event handler of the attached property you will set the EmptyContent property directly.
I will log this for an investigation so that we can determine if this is indeed a bug.
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