Before proceeding, let me take a moment to tip my hat to Splunk for its dashboard editor. There are only a couple of reasons why you would still need to edit simplified XML dashboards: forms and post-processing data. I predict that these reasons will go away in the future as more features are added to the dashboard editor.

The documentation for simplified XML panels can be found by searching http://www.splunk.com/ for panel reference for simple XML. The following link is a good place to start:

http://docs.splunk.com/Documentation/Splunk/latest/Viz/PanelreferenceforSimplifiedXML