The ornamental design for a displayscreen with a graphical user interface, as shown and described.
1. Display screen with a graphical user interface
1.1 : Front
1.2 : Top
1.3 : Right
The dot-dash-dot broken lines represent a display screen and form no part of the claimed design. The broken lines within the display screen represent portions of the graphical user interface and form no part of the claimed design. The broken lines represent environment and form no part of the claimed design.