As you mentioned the RadEditor can be used only with simple HTML markup. Note that this markup is used by the browser to render the implemented code.
The RadChart or RadHtmlChart are ASP.NET controls and implemented via the corresponding markup. Note that the rendering of the back-end HTML code generated by the ASP.NET code is done via IIS. Using the Telerik Editor the code is designed directly in the browser and server logic cannot be applied.
Note that the rendered code of the Chart is SVG and the only available one that could be eventually used in the RadEditor to modify the source. Although I assume such task would be much more complex for the users.
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