Free magnetic energy – relative helicity diagram for synthetic (lower left) and observed (upper right) ARs by means of their volume-calculated (red symbols) and surface-calculated (blue symbols) budgets. Eruptive (non-eruptive) AR cases are denoted by diamonds (circles). The respective least-squares best fits are denoted by the thick orange (observed ARs) and green (MHD-simulated ARs) lines. Gray symbols denote previous results on the EH diagram with asterisks corresponding to ARs (Tziotziou et al. 2012) and squares corresponding to quiet-Sun regions (Tziotziou et al. 2014); their respective least-squares best fits are denoted by solid black lines. The dotted horizontal and vertical lines correspond to the previously reported (Tziotziou et al. 2012) thresholds of free magnetic energy (4 × 10³¹ erg) and relative magnetic helicity (2 × 10⁴² Mx²) for ARs to enter eruptive territory.