An X-ray or image unit (4, 6, 10) creates an anatomical reference image of a patient's heart (2). An electrode catheter (30) with at least one electrode (24) senses intracardial electrical activity, feeding into the heart. A signal processor (34) determines activation times by sensing electrical activity in relation to a reference time for electrical activity at different points in the heart. The resulting graphic image shows the activation times at different points in the heart and can be laid over the anatomical image. Reference information allows a reference point to be entered in the anatomical image. With the help of reference points control information (28) allows the graphic image to be shifted in relation to the reference image.
