Like in Baudline or Adobe Audition 3
Please don’t file feature requests here. They belong in the issue tracker. This place can sometimes be useful if its an idea that needs a litle discussion before filing it.
Also, this is the kind of feature you attach a price to. Its a major piece of work (very far from trivial), and unless someone is bored out of their minds and is very attracted by this idea, it will not be implemented unless somebody pays for it.