Methods, systems, and apparatus, including computer programs encoded on a computer storage medium, for displaying an electrocardiogram (ECG) on a computing device using actions including receiving data corresponding to the ECG, processing the data to generate a plurality of traces, displaying a plurality of trace display windows, displaying each trace of the plurality of traces in a trace window of the plurality of trace windows, receiving user input corresponding to at least one trace window of the plurality of trace windows, and modifying a display of each trace of the plurality of traces in the respective trace display windows in response to the user input.