Currently you can't provide more than one Label per Series Item. However there is a workaround for your scenario:
You can create third serie , for example a Line Serie. Make it's Stroke Transparent cause all we'll need is the Labels of the DataPoints. You'll also need to set the Fill and Stroke of the PointMark to be transparent.
To be able to show percentages as Items Labels you may use the following format expression
You may find a sample project attached that demonstrates what I mean.
All the best,
the Telerik team
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