According to Boyle's law, $P \propto \frac{1}{V}$ at constant temperature, so each isotherm is a rectangular hyperbola. At a given pressure, $V \propto T$, so the isotherm for the higher temperature lies farther from the axes: 600 K above 400 K above 200 K. [add image: P vs V hyperbolas with 600 K outermost and 200 K innermost]