The attached workbook has been made with random data to demonstrate the issue. Whichever fruit has been selected (via a parameter) displays as an outlined circle, whereas all other fruit appear as filled circles. I would like to display an annotation on whichever fruit is selected. That would mean the annotation needs move to the correct item whenever the selection is changed. I have no idea how to do this. Note, the dual axis is already in use displaying some other info, so I can't use that as a workaround.
Below are some images that may help explaining the functionality I want:
Annotation on selected item:
User changes the selected item via parameter (annotation appears on the newly selected item):
Any help would be appreciated,
Thanks!

I don't believe the annotation can be truly dynamic in the way that you are hoping. You can, however, use a label on the text mark that will be dynamic and will only label the selected mark (open circle). I mocked up the following. If you think this will work, then please read on.
Create three different calcs, each of them ensuring that the fruit in the viz matches the fruit in the parameter. If it does, then show the label and if it does not, then yield a null:
@Label Selected Fruit
IF [Fruit] = [Select fruit] THEN [Fruit]
ELSE NULL
END
@Label SomeXValue
IF [Fruit] = [Select fruit] THEN 'someXvalue: ' + STR(INT([someXvalue]))
ELSE NULL
END
@Label SomeYValue
IF [Fruit] = [Select fruit] THEN 'someYvalue: ' + STR(INT([someYvalue]))
ELSE NULL
END
Add those to the marks card on the shape axis. I knocked down the opacity on the color a bit as well. Now, that label will only appear on the mark in the parameter.
Attached is a workbook. Let me know if this helps 😄 .
Thanks,
Kevin Flerlage