Control and output-quality variables of a process plant (1) are plotted against parallel axes in a display unit (7). Convex hulls between pairs of variables are calculated from sets of the variable-values accumulated historically in stores (7, 9) during successive runs of the process, and hulls (HH; TC, BC) between successive axis-pairs are displayed. New variable-values for process optimization are fixed for the variables taken in turn, each selection being made within displayed ranges (Rn-Rn) derived from the hulls effective between the respective variable and the variables already fixed. A display unit (13) provides on-line parallel-axis display of variable-values from the plant (1), showing alarm carets (DC, UC) where values violate limits (UL, LL) determined by the convex hulls, and allowing variation in the displayed-value for observing the resultant effect in avoiding alarm situation and towards optimization.
