There is one possible scenario, which would lead to such behavior. It involves AJAX updates, where other parts of the page are updated, but not the chart. Is this the case?
Here are some details about how RadChart works: the RadChart server control renders an image and places it in the session or saves it on the disk. Later, when the browser requests the image, the ChartHttpHandler retrieves the image and deletes it from the session/disk. Now, when AJAX request is performed, a new image is generated as the page lifecycle is preserved, but that image is never requested by the browser, so it stays in the session.
1. The first solution is to re-work the ajax settings, so that RadChart is always updated on AJAX request. You can try adding all the RadCharts in the page as updated controls in every item in RadAjaxManager.AjaxSettings collection. For more information, please read this
2. As I said earlier, the ChartHttpHandler automatically deletes the unnecessary images, so you shouldn't need to manage this yourself. If the image files remain after all, this means that there is something that is preventing the RadChart from deleting them. If this is the case, please check if the files are getting locked.
I hope this helps.
All the best,
the Telerik team
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