In the case of parallel plate capacitors, such as the one featured in the simulator, the electric field between the plates and near the center of the plates can be considered approximately constant for a given voltage and separation distance.
What is the relationship between the electric field (E), the voltage (V), and the distance (d) between the two plates?
V=Ed
E=Vd
V=E$^2$/d
V=E/d$^2$