Total gate charge of a capacitor variation with voltage?
MOSFET's operate in three distinct regions: cutoff, saturation and triode. The region of operation of the MOSFET depends on the gate-source and drain-source voltages. The gate capacitance of the MOSFET changes for each mode of operation.

For the cutoff region:
$$
C_{gs} = WC_{ov}
$$

For the saturation region:
$$
C_{gs} = \frac{2}{3}WLC_{ox} + WC_{ov}
$$

For the (deep) triode region:
$$
C_{gs} = \frac{WLC_{ox}}{2} + WC_{ov}
$$

The capacitance is quite low at the cutoff region, since only the overlap capacitance (the area of the device where the gate overlaps the drain and the source terminals) of the MOSFET contributes. In the saturation region, where the channel begins to form, the capacitance is greatly increased, and in the deep triode region, it decreases, to a value greater than the cutoff regime, but lesser than the saturation regime. In your graph, the derivative of Q with respect to V, yields the capacitance value. However, it's worth noting that this change in gate capacitance is not discontinuous as your graph shows.

The derivation of these figures require quite a bit of understanding in device physics, far beyond my capabilities of explaining, but a great resource is here if you're interested: http://www.eecs.berkeley.edu/~hu/Chenming-Hu_ch5.pdf