Renderer for Dia files
This plugin will render a Dia attachment to PNG and insert the rendered file into the page.
Note: This plugin is alpha quality and will probably light your machine on fire. On Trac 0.11 and later it just won't work at all without modifications.
If you have any issues, create a new ticket.
Download the zipped source from here.
Source is attached. You will need to edit the source to reflect your environment (i.e. where dia is located). See TODOs in the code. Install in $YOURTRACPATH/plugins.