Left panel: a visual representation of the analytical model (green curve, Eq. (12)) used to fit the emission features of the PV diagrams, applied to the reference model at an inclination of 36^◦. The slope of the curve around v = 0 km s^-1 (white dashed line) determines the characteristical skew of the diagram. Right panel: relation between the measured skew of the PV diagram and the inclination angle.