Based on the supplied information, it is hard to determine what is causing this unwanted behavior. However, you can try to clear the RadChart.Series and rebind the chart after you change the DataSource. In cases, where you don't need grouping, you can clear the data grouping related properties, because the chart will not do this automatically. For example:
radChart.DataSource = newData;
If the issue persists, could you please elaborate a bit more on the way you setup the RadChart control and implement this functionality? We also appreciate if you open a formal support ticket, and send us a sample runnable project with dummy data, that demonstrates the unwanted behavior. We will review it locally, and advise you further.
the Telerik team