Display Screen or Portion Thereof with a Graphical User Interface

The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. FIG. 1 is a front view of a first embodiment of a display screen or portion thereof with a graphical user interface showing our new design; FIG. 2 is a front view of a second embodiment thereof; FIG. 3 is a front view of an embodiment thereof; and, FIG. 4 is a front view of an embodiment thereof. The alternating long-short dashed broken lines forming a frame around the graphical user interface are for the purpose of illustrating a display screen or a portion thereof that forms no part of the claimed design. The out-of-focus or blurred text within the graphical user interface forms no part of the claimed design.