Use self-contained benchmark for main chart

[Bobronium]

No description provided.

[codspeed-hq]

CodSpeed Performance Report

Merging this PR will degrade performance by 22.47%

_{Comparing better-charts (87623aa) with main (ba3b7d6)¹}

Summary

❌ 1 regressed benchmark
✅ 357 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
❌	test_individual_cases[copium-cpython:91610:repeating_atomic]	22.9 µs	29.5 µs	-22.47%

Footnotes

1. No successful run was found on main (7a58f61) during the generation of this report, so ba3b7d6 was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩