I claim the ornamental design for a display panel of a programmed computer system with a graphical user interface, as shown and described.
The sole FIGURE is a front view of a display panel of a programmed computer system with a graphical user interface.
The outermost broken line rectangle showing of a portion of a display panel is for the purpose of illustrating environmental structure and forms no part of the claimed design. The remaining broken line showing of portions of the graphical user interface within the display panel forms no part of the claimed design.