A medical system, comprising: a catheter configured to be inserted into a chamber of a heart of a living subject, wherein the catheter comprises catheter electrodes configured to contact tissue at respective locations within the chamber of the heart, a shaft having a distal end, and a distal end assembly on which the catheter electrodes are disposed; a display; and processing circuitry configured to receive signals from the catheter, and in response to the signals: sample voltage values of the signals at respective sampling times; compute respective curved three-dimensional surfaces describing electrical activity of the tissue over the catheter electrodes at respective ones of the sampling times, responsively to: (a) respective projected positions of the respective catheter electrodes; and (b) the respective sampled voltage values indicative of electrical activity of the tissue that is sensed by the respective catheter electrodes at the respective locations at the respective sampling times; and render the respective three-dimensional surfaces to the display over time, characterised in that the respective projected positions of the respective catheter electrodes are projected onto a plane perpendicular to an axis of the shaft. a catheter configured to be inserted into a chamber of a heart of a living subject, wherein the catheter comprises catheter electrodes configured to contact tissue at respective locations within the chamber of the heart, a shaft having a distal end, and a distal end assembly on which the catheter electrodes are disposed; a display; and processing circuitry configured to receive signals from the catheter, and in response to the signals: sample voltage values of the signals at respective sampling times; compute respective curved three-dimensional surfaces describing electrical activity of the tissue over the catheter electrodes at respective ones of the sampling times, responsively to: (a) respective projected positions of the respective catheter electrodes; and (b) the respective sampled voltage values indicative of electrical activity of the tissue that is sensed by the respective catheter electrodes at the respective locations at the respective sampling times; and render the respective three-dimensional surfaces to the display over time, characterised in that the respective projected positions of the respective catheter electrodes are projected onto a plane perpendicular to an axis of the shaft. sample voltage values of the signals at respective sampling times; compute respective curved three-dimensional surfaces describing electrical activity of the tissue over the catheter electrodes at respective ones of the sampling times, responsively to: (a) respective projected positions of the respective catheter electrodes; and (b) the respective sampled voltage values indicative of electrical activity of the tissue that is sensed by the respective catheter electrodes at the respective locations at the respective sampling times; and render the respective three-dimensional surfaces to the display over time, characterised in that the respective projected positions of the respective catheter electrodes are projected onto a plane perpendicular to an axis of the shaft. <EOS>