A method includes receiving an electrocardiogram (ECG) measured at a given location over a portion of a heart. Based on the measured ECG, a rhythmic pattern is identified over a given time-interval. The rhythmic pattern corresponds to a relation between a present cardiac cycle length and a preceding cardiac cycle length. Based on the identified rhythmic pattern, a classification of the location as either showing regular pattern or showing arrhythmia is determined. The location is graphically encoded according to the classification. The graphically encoded location is overlaid on an anatomical map of the portion of a heart.
