Why is capacitor a linear element?
First of all, an I-V curve does not make any sense for a capacitor. This is because a capacitor follows the following equation:
$$i = C \frac{dV}{dt}$$

Note that the current depends on the rate of change of voltage. So you can have the same current at two different voltages, if the rate of change is the same. 

The reason a capacitor is a linear device is because differentiation is linear. Superposition becomes:
$$i_1 +i_2 = \frac{d}{dt}(v_1 + v_2) = \frac{dv_1}{dt} + \frac{dv_2}{dt}$$