I am unsure what is the final result that you are after. The RadChart does not have out-of-the-box functionality to show a label for a certain item. The chart shows either all labels or none.
Perhaps you can work this out, but the solution would highly depend on your set up. One way to go is to use the selection API of the chart (chart1.DefaultView.ChartArea.SelectItem and/or other methods) to select an item and handle the visibility of the label. You will need mainly to use code behind.
Another approach would be to use the hover effects to annotate an item. I have attached an item to demonstrate this.
Let us know if you were able to achieve the desired effect. If you need further assistance I will ask that you attach a snapshot of the desired effect, so that it is clear what you are after.
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