This issue is not directly related to the RadHtmlChart control but the select parameter of the SqlDataSource. As long as the right data source is passed to the chart it is supposed to render it properly.
You can make a test by replacing the RadHtmlChart with an ASP control (e.g. GridView). If the issue is still reproducible then it is not related to the chart control and possible cause must be searched somewhere else (e.g., in the select parameter of the SqlDataSource).
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