docs: fix graphs on RTD #196

Job	Run time
Tests on paywalled platforms (macos-13-xlarge, 3.12)	49s
Tox test (py38)	1m 58s
Tests on other platforms (windows-2019, 3.10)	2m 56s
Tox test (py39)	2m 2s
Tests on other platforms (windows-2022, 3.11)	2m 44s
Tox test (py310)	2m 1s
Tests on other platforms (windows-2022, 3.12)	2m 34s
Tox test (py311)	1m 56s
Tests on other platforms (macos-12, 3.11)	2m 57s
Tox test (py312-cover)	3m 3s
Tests on other platforms (macos-13, 3.12)	2m 7s
Tox test (docs, graphviz)	1m 53s
Docker image	1m 1s
PyPI packaging	0s
	28m 1s

Provide feedback