Display screen or portion thereof with a graphical user interface

[0001]The FIGURE is a front view of a display screen or portion thereof with a graphical user interface showing our new design;

[0002]The medium weight evenly spaced broken lines around the FIGURE show a display screen or portion thereof and form no part of the claimed design. The light weight evenly spaced broken lines within the FIGURE show portions of the graphical user interface that form no part of the claimed design. The dot-dash broken lines within the FIGURE show portions of the graphical user interface that form no part of the claimed design. The thick evenly spaced broken lines within the FIGURE show portions of the graphical user interface that form no part of the claimed design. The convergence of the aforementioned various broken lines at the center of the circular chart forms no part of the claimed design.