"0" dB on a mic plot is usually chosen based on the mic's output at 1,000 Cycles (1 kHz). The output will usually go up and down from there. Usually, the output will start to drop off below 1,000 Hz, or stay fairly smooth till it reaches it's cutoff point.
Above 1,000 Hz, it gets a little weirder, since resonances, reflections, and cancellations will affect it, and you'll get various peaks and dips until the mic reaches its high end cutoff point where it starts to drop like a rock.
The problem in judging the mic's perfomance is difficult, since some sources will hit right where the mic has a peak, while another source may miss all the peaks entirely.
Plus a lot of manufacturers "fudge" the response (or show an "average response", so it's almost impossible to really tell how one particular mic is going to work on one particular source.