Aha yes, took me ages to find that one. When you're in dislpay mode (i.e. looking at the graph or gauge view or whatever), hold the up button for 2 seconds. That resets all the peak values.
Took me a while to figure out the record and replay too. To record you press the down button, it should say "rec" at the top. Then you press down again to stop recording - it should beep. To play back you press the set button , i.e. the middle of the knob (ooerrr mrs!) for 2 seconds. Then you can press it again to pause, and turn it to go back and forwards. You press it again for 2 secs to go back to normal mode.
Although it only displays three things at once, it records the whole lot, so you can select another 3 things to look at and replay it again.