A conducting bar of mass \(m\) and length \(\ell\) slides on two parallel, frictionless horizontal conducting rails separated by a distance \(\ell\) in a uniform vertical magnetic field of magnitude \(B\). The left ends of the rails are connected to an unknown resistor of resistance \(R\), while the electrical resistance of the bar and rails is negligible. At time \(t = 0\), the bar is given an initial speed \(v_0\) to the right and subsequently slows down due to magnetic braking. To determine the resistance \(R\) from a linear graph, which quantity should be plotted on the vertical axis versus time \(t\) on the horizontal axis, and what is the resistance \(R\) in terms of the magnitude of the slope \(s\) of the resulting best-fit line?