The chart always represents air at a constant atmospheric pressure (typically sea level at 14.7 psi or 101.325 kPa). Different charts exist for high-altitude locations.
The "normal" air temperature measured by a standard thermometer. psychrometric chart how to read
To plot a state point, you generally need two known properties (e.g., Dry Bulb Temp and Relative Humidity). The chart always represents air at a constant