In an experiment to measure electrostatic interactions, two small charged spheres, Sphere 1 with positive charge \( +q \) and Sphere 2 with positive charge \( +4q \), are held at various center-to-center separation distances \( r \). The magnitude of the repulsive electrostatic force \( F_E \) between the spheres is measured and plotted as a function of \( \dfrac{1}{r^2} \), yielding the linear best-fit line shown in the graph. Based on the graph, what is the magnitude of the charge \( q \) on Sphere 1?