Yeah to go a little more in depth, and because I like to beat dead horses...but only on Mondays, they don't expect it on Mondays....Anyway, yeah the negative numbers are how much negative pressure *read suction* the pistons are causing on the manifold. At idle its neither a good idea nor necessary for the supercharger to be pounding air into the manifold, you only need this when you're accelerating. At the inlet of the supercharger you have another valve you can't see that looks like your throttle body valve only smaller. At idle or cruising this valve opens and lets air around the supercharger so it never gets compressed, which is why it stays below 0. This is roughly how much power your car would have if you ripped the supercharger off, sad huh? Right now this gauge is handy in diagnosing engine problems. If it only read 0+ you wouldn't have the benefit of possibly figuring out what is wrong with your car. I'm only recently reading into this but here are a few examples:
Intermittent Fluctuaiton at idle - Ignition miss, sticking valves, lifter bleeding off (hydraulic), or just a BIG camshaft
Gradual Drop and Idle - Clogged exhaust, excessive backpressure (extreme cases engine will die at idle)
Mainly all you can use for diagnostics is at idle, above that and the diagnosis is skewed by the supercharger ramming air into the manifold and smoothing out the needle. Just my thoughts though.
Stuff got done.