A medical device system and associated method for guiding ablation therapy sense cardiac signals using implantable electrodes and detect spontaneous cardiac events from the sensed cardiac signals. Pacing pulses are delivered in response to detecting a spontaneous cardiac event and a return cycle length is measured. The spontaneous cardiac event is clustered with a previously detected cardiac event in response to the measured return cycle length. Data corresponding to the clustered cardiac events is displayed to guide an ablation therapy.