To see a working example of this, take a look at StoryBoard Download the 30 image from https://ci.inria.fr/pharo-contribution/job/StoryBoard/ Start it, open the seaside control panel and start a ZnZincServerAdaptor on port 8080. Open a web browser on http://localhost:8080/story, register yourself, add a project and start adding stories. They can be dragged and dropped.
The actual code is in the rendering methods of SBProjectStoryColumn Stephan
