The ornamental design for a displayscreen or portion thereof with a graphicaluserinterface, as shown and described.
FIG. 1 is a front view of a display screen or portion thereof with a graphical user interface, showing our new design, according to a first embodiment thereof; and,
FIG. 2 is a front view of a display screen or portion thereof with a graphical user interface, showing our new design, according to a second embodiment thereof.
The broken line showing of a display screen with a graphical user interface is included for the purpose of showing portions of the article which form no part of the claimed design.