Space-charge-filled parallel-plate capacitor ($V^{\prime \prime }=-\rho /{ϵ}_0$)

Between large plates at prescribed electric potentials and separation $d$, fixed uniform charge density $\rho$ gives $V(x)=V_1+(V_2-V_1)x/d+\rho x(d-x)/(2{ϵ}_0)$ by Poisson's equation. The signed electric field is linear. Its magnitude can have an interior V-shaped minimum, so a sketch of signed field alone need not answer a request for field magnitude.