The object of the invention is to provide a means for quickly determining a gradient in a diagram depicted on a screen used in the control of an industrial process. According to the invention, at least one rule (8, 9, 10) is superimposed on the diagram (7). This rule can be positioned on said diagram (7) by means of the controlled screen elements (14). The gradient of the ruler is then automatically displayed in the desired physical units, in a display field (4) (gradient ruler).
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