The unexpected behavior you are having is new to us. I have tried to reproduce it with a SQL datasource but to no avail. You can watch a short video with the test here
From the provided snap shot, it seems that the PieSeries has six items - five items are displayed properly, while the sixth one has just a label that gets out of the dark green item. It might be possible that this sixth item has an invalid YValue or Name. For example if the last item has '<' symbol in its name, this will reproduce your issue.
that shed more light on the matter.
You can also inspect what data has been passed to the RadHtmlChart by right clicking on the page and then select ViewPageSource. In the $create() statement, located at the bottom of the page source, are stored the settings of the HtmlChart. You can investigate the "_dataSource" object which stores the data from the datasource. I have also demonstrated how to check that at the end of the recorded video.
the Telerik team
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