We are not aware of such error and we were not able to reproduce it on our side. It is possible to be caused by the Visual Studio caching mechanism. You can try to resolve it by manually deleting the "bin" and "obj" local folders of your project and then Clean and Rebuild it.
Also, keep in mind that RadChart is our old charting control which has many known issues and limitation. If you are just starting to use a chart in your application or the RadChart's integration is not too complex, we recommend you to use the new RadChartView
suite instead. The new controls have better implementation and improved performance. Furthermore, they are very flexible and easy to setup. You can read more about the differences between the charts in the RadChart vs. RadChartView
Please give the new suite a try and let me know if it works for you.
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