From Fig.4 can see that the curve is obviously a quadratic curve. But, their values is relatively large that 55555 when the voltage is just 60 V. Now consider 00 ( ) xu C C u , is inappropriate when u0 is 30 V. The curve in Fig.4 is just can represent the change that other voltage values relative to 30V. It is necessary to get the absolute voltage coefficient curve as the capacitor is impossible always calibrated at 30 V.
So, assume the voltage coefficient of 30 V is , can get