Upper panel: model star-formation rate surface density as a function of the model molecular gas surface density. The symbols are the same as in Fig. 10. The dotted lines are linear and square fits to guide the eye. The dashed lines are robust fits to the different galaxy samples. Lower panel: model star formation per unit area as a function of the model molecular gas surface density multiplied by the angular velocity (divided by the dynamical timescale). The dashed line represents a robust bisector fit to the entire sample with its associated rms (dotted lines).