I am unsure what is the desired output you are after. Perhaps you need a line that is always parallel to the vertical axis? If this is the case, the CartesianGridLineAnnotation is a better fit, as it does this by itself.
If this is not what you are looking for, I will ask that you provide us with a couple of snapshots demonstrating the desired visual presentation for the custom line annotation, so that we know how to help. Perhaps you can attach an event handler to the DataBindingComplete event of the series and use some custom code there. The solution might depend on the actual results you require. Send us a couple of snapshots (drawings of some sort) so that we can resolve this as quickly as possible.
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