I reviewed your code and it looks correct. I tried to reproduce the problem locally using your code, but to no avail. In case I understand the problem correctly, the background image of the chart is not loaded correctly. Do you get this result on initial page load, or after an AJAX request? Could you please try disabling the AJAX to see if this will make a difference? Additionally, you can check the status of the requests, using Firebug (the Net tab). Another thing that you can test is whether you can reproduce the problem on another machine - just to make sure that the problem is not in your concrete browser configuration or a plugin that you have installed.
In case these suggestions do not help you find the cause for the problem, please open a formal support ticket and send me a running test project that demonstrates the problem.
the Telerik team
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