Try setting InitialDelayTime for the RadAjaxLoadingPanel. The InitialDelayTime property specifies the delay in milliseconds, after which the RadAjaxLoadingPanel
will be shown. If the request returns before this time, the RadAjaxLoadingPanel will not be shown.
Check out the following help document for more on this. RadAjaxLoadingPanel.
This does not fix the issue.
The issue is not with the RadAjaxLoadingPanel but it is with the loading image within the RadChart control when a transparent skin like Vista is being used.
You can fix it by doing this
<style type="text/css"> .rchImgWrap div { background: url() !important; } </style>