In an experiment to determine the mass-to-charge ratio \(m/q\) of positive ions, ions of mass \(m\) and charge \(q\) are accelerated from rest across an adjustable potential difference \(\Delta V\). The ions then enter a region containing a uniform magnetic field of known magnitude \(B\) directed perpendicular to their velocity, which deflects them into circular arcs of radius \(r\). To obtain a linear graph from the experimental data, which quantities should be plotted on the vertical and horizontal axes, and how is the mass-to-charge ratio determined from the slope \(S\) of the resulting best-fit line?