P,V,T graphs are 3D graphs that looks like the surface shown. For a fixed mass of gas, the possible P,V,T lies on the surface. The surface may be represented by the pV graph plus lines representing temperatures called isothermals ( constant temperature curves) It would be easier to figure out where you should draw a point leading to a higher temperature. Imagine if I give you a map without contours showing the height above sea level , you could underestimate the time you need to travel from one point to the next point without knowing whether it is uphill. Therefore, always draw faint lines to indicate isothermals so that you can draw your processes with arrows going in the right direction in case there is increase or decrease in temperature.
