This is a plot showing the output bandwidth. So the scale on the horizontal axis does not relate to the original carrier frequency. It is telling how fast the device can respond to changes at the input. It's not measured down to dc but I suppose you could do this. To create this plot we AM-modulate at carrier and measure the pp output voltage (which will come out on Venv at the frequency of the am signal). So the device could be considered to be an am-detector. Like I say we started at 2 MHz but we could have started at dc (i.e. no am modulation). In that case, we would be applying a steady state sine wave and would measure 0Vpp sitting on top of a dc level that is proportional to the amplitude of the un-modulated carrier.
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