I really like the Flot Pie Chart plugin that came with the APEX sample applications (see this blog post in which apps this plugin is used).
But when I wanted to make changes to it, I couldn't... the PL/SQL Code was read-only.
To make the source read/write again you need to delete the reference to the Master Plug-in.
If you export the plugin and open it in your text editor search for "p_reference_id" and set that to null (instead of the number).
,p_reference_id => null
In case you wonder why I want to change that plugin; the plugin is great, but there's a newer version available of the Flot Charts which holds some nice improvements and the current plugin is not mobile compatible and responsive.