In order to assist in evaluating/monitoring pain, a touch-sensitive graphics tool (TG) is provided with a local application (TGapps) for displaying a graphical representation of a patient's body, on which the patient can delimit a zone. A control application cooperates with the local application (TGapps) in order to record each delimited zone together with the corresponding surface data. The control application is configured to control a patient auto-calibration function that can determine deviations between a plurality of target points designated by contact on the patient's physical body and the corresponding points that the patient points to on the graphical representation displayed on the screen. The delimitation by the patient of a zone on the graphical representation is altered according to the patient auto-calibration deviations.