Unfortunately the powertap computer doesn't support that. It only shows avg power for the entire ride.

Plus, say you're doing a 10-min interval, and you're at minute 8. You can drop significantly below your target power, and the average will only reflect it by 1 watt, after about 15 seconds or so. So it seems like you'd be doing alot of work above and below your target, instead of right on it.