If thats the case, drag events are not relevant.
You will need to override mousePress, mouseMove and mouseRelease events of the scene.
You can contruct a polygon out of the move events, and add a item as u like to the scene.

Also if only drawing is your aim, you wont even need graphics scene or view. You can simply draw on a widget. See Scribble example under Widgets in Qt Demo.

Hope this helps