As we look at your code and the picture it seems that the TrackballInfoTemplate is not applied and the series is using its default template.
Also, keep in mind that the DataContext passed in the template is not the view model of the data point, but an object of type DataPointInfo. So, in order to get your view model you can use the following syntax:
You can also take a look at the attached project where this approach is demonstrated.
Please let us know if this solution works for you.
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