QGraphicsView

[marcel]

You also have to add set a scene to the view.
Then you can use QGraphicsLineItem to draw a line in the scene.

Search the forums for QGraphicsView. There are a lot of examples.
Also check the Qt Demos and read about the Graphics View Framework in the Assistant( or the online docs). You can learn a lot from there and you shouldn't start doing any drawin until you have read those.

Regards