I don't know at which point does the clipping occur ...
From the facts it's easy to say: at the wrong one. That's what a bug report is good for.
(btw. clipping is supported on graphics view if you set a proper hint on an item), ...
AFAIR the line was painted by QPainter from a homebrew item. In such a situation the hint gets lost in the GV framework and doesn't make it into the painting operation.
With lines it is relatively easy to clip, but if you have a complex shape, you can't just guess where to clip.
No, it's also easy to clip polygons ( f.e. Sutherland-Hodgman ) and most (all ?) shapes are ( or can be ) build from polygons. Believe me - I had to implement this all in Qwt.
The problem in this particular application is the wrong calculation of line coordinates. The scene is more or less 1000x1000, so having coordinates that range up to several thousand in both positive and negative values yields an item that is more or less 1000 times bigger than the whole scene.
Well, if you implement zooming you often run in similar situations.

Uwe

PS: Antialiasing of vertical lines is a nop. No surprise, that the performance is much better then.