gh-36162: minor details in graph_plot
just fixing a few little things in the modified file
### :memo: Checklist
- [x] The title is concise, informative, and self-explanatory.
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URL: https://github.com/sagemath/sage/pull/36162
Reported by: Frédéric Chapoton
Reviewer(s): Frédéric Chapoton, Kwankyu Lee
