Display Screen or a Portion Thereof with a Graphical User Interface for a Cardiac Mapping System

The sole FIGURE is a front view of a display screen or a portion thereof with a graphical user interface for a cardiac mapping system, showing our new design.

The outermost broken-line rectangle at the periphery of the sole FIGURE shows a display screen or portion thereof and forms no part of the claimed design. The remaining broken lines show portions of the graphical user interface and form no part of the claimed design. All subject matter depicted in the sole FIGURE other than the four solid-line squares, one solid-line large circle, and seven solid-line small circles at the upper right corner of each of the four solid-line squares is is excluded from the claimed design.