The curve sweeps accelerating voltage at the current mass-flow rate; the dot marks the live operating point. Raising voltage raises Isp but lowers thrust for the same mass flow — the classic high-Isp/low-thrust trade-off of electric propulsion.
Physics: vₑ = √(2qV/m), F = ṁ·vₑ, Isp = vₑ/g₀. A single-ionised (q = e) atom of the chosen propellant is accelerated across the voltage V; the lighter argon ion reaches higher velocity than xenon at the same voltage, but xenon's higher atomic mass gives more thrust per ion for a fixed mass-flow rate.