Thank you for the provided details.
I have noticed that the example link you have send is outdated. It refers to the previous UI of the documentation. I can assure you that this will be fixed. The Chart Annotations article was reviewed and updated. You can check it at the link below:
Regarding to the Invalid cast exception, I have created a sample and I could not reproduced the issue on my side. Please find my test project attached. Download it and give it a try.
If the issue still observes I would like to ask you to modify the attached project regarding your setup and send it back to me? In this way we could research the case further. Note that you should open a support ticket and attach the file there because in the forum only image attachments are allowed.
I hope I was helpful.
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