The ornamental design for a displayscreen or portion thereof for a graphicaluserinterface, as shown and described.
FIG. 1 is a front view of a display screen or portion thereof for a graphical user interface showing my new design; and,
FIG. 2 is a front view of a second embodiment which illustrates an example environment.
The long and short dashed broken line showing of a mobile terminal in FIG. 1 forms no part of the claimed design. The mobile terminal, display screen, and the text/words, numbers, symbols and images of the graphical user interface shown in broken line in FIGS. 1 and 2 depict unclaimed subject matter and form no part of the claimed design.