I am not sure that I understand you very well. Perhaps you can use the approach in the attached project. You can create a custom utilities class with attached properties and ensure that an annotation is in the visible area while zooming.
I can suggest other solutions as well. Another would be not to use annotations but instead use your own visuals. You can place the visuals in a Canvas and lay the Canvas over the chart. Then you can find the coordinates for a value (DateTime value) from the conversion api of the chart (ConvertDataToPoint method). After you find the absolute position (take into consideration the PanOffset) you can set the Left and Top of the custom visual.
Both of these approaches do not require that you use unbound mode. And I suggest you adopt one of them, because I do not see a very good reason not to use the chart data binding capabilities.
The DataItem is set internally and is only set when the chart is in a databound mode. Let us know if we can assist you further.
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