Provided are controllers, methods and systems for monitoring cardiac data for the presence of arrhythmia. A controller as provided comprises input means arranged to receive first cardiac data corresponding to a time period, processing means and output means. The processing means is arranged to identify, within the first cardiac data, a plurality of events corresponding to ventricular contraction, and a time associated with each event； determine a plurality of intervals between the times associated with chronologically successive events； and produce second cardiac data in dependence on the determined intervals. The output means is arranged to transmit an output signal to a display means based on the second cardiac data, for displaying a beat-to-beat display plot corresponding to at least a part of the time period on the display means. A system as provided may comprise such a controller and a display means for displaying the beat-to-beat display plot corresponding to at least a part of the time period. Also provided are methods of analysing and monitoring cardiac data for the presence of arrhythmia, and non-transitory, computer-readable storage media which store instructions thereon that when executed by one or more processors causes the one or more processors to carry out such a method.
