graph.py: remove extraneous 'f' prefix

[cakebaker]

This PR fixes the following issue shown by the linter (see https://github.com/uutils/coreutils-tracking/actions/runs/21522125807/job/62015936175?pr=37#step:5:12):

F541 [*] f-string without any placeholders
   --> graph.py:120:11
    |
119 | # Create text box
120 | textstr = f"Latest Results:\n"
    |           ^^^^^^^^^^^^^^^^^^^^
121 | textstr += f"Pass: {pass_pct:.1f}%\n"
122 | textstr += f"Fail: {fail_pct:.1f}%\n"
    |
help: Remove extraneous `f` prefix

Update: the PR also formats graph.py.