You can customize your labels forma by using DefaultLabelValue property like this for example:
series1.DefaultLabelValue = #%;
"#%" is used for a percentage of the total sum (of all items).
Alternatively you could use:
series1.DefaultLabelValue = "#Y" + "%";
which will add "%" to your series items labels.
Further information can also be found here:
Hope this helps.
the Telerik team
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