There are three observations that can be made when looking at a generalized compressibility chart.  These observations are:
# Gases behave as an ideal gas regardless of temperature when the reduced pressure is much less than one (P ≪ 1).
# When reduced temperature is greater than two (T > 2), ideal-gas behavior can be assumed regardless of pressure, unless pressure is much greater than one (P ≫ 1).
# Gases deviate from ideal-gas behavior the most in the vicinity of the critical point.