1. A system comprising: a non-transitory memory to store electrical signal data representing electrical signals for a plurality of points distributed across a surface on or within a patient's body and machine-readable instructions; and a processor to access the non-transitory memory and execute the machine-readable instructions to at least: select at least a portion of the plurality of points across the surface; calculate an activation vector for each of the selected plurality of points across the surface based on time delays among respective sets of neighboring signals across the surface according to the electrical signal data for the selected plurality of points; calculate an activation time for each of the selected plurality of points based on the calculated activation vectors; and provide an output to display a graphical image for the at least the portion of the plurality of points based on the calculated activation times.