To cite ggtreeExtra in publications use:

  Guangchuang Yu. (2022). Data Integration, Manipulation and
  Visualization of Phylogenetic Trees (1st edition). Chapman and
  Hall/CRC.

A BibTeX entry for LaTeX users is

  @Book{,
    title = {Data Integration, Manipulation and Visualization of Phylogenetic Treess},
    author = {Guangchuang Yu},
    publisher = {Chapman and Hall/{CRC}},
    year = {2022},
    edition = {1st edition},
    url = {https://www.amazon.com/Integration-Manipulation-Visualization-Phylogenetic-Computational-ebook/dp/B0B5NLZR1Z/},
  }

  S Xu, Z Dai, P Guo, X Fu, S Liu, L Zhou, W Tang, T Feng, M Chen, L
  Zhan, T Wu, E Hu, Y Jiang, X Bo, G Yu. ggtreeExtra: Compact
  visualization of richly annotated phylogenetic data. Molecular
  Biology and Evolution 2021, 38(9):4039-4042. doi:
  10.1093/molbev/msab166

A BibTeX entry for LaTeX users is

  @Article{,
    title = {ggtreeExtra: Compact visualization of richly annotated phylogenetic data},
    author = {Shuangbin Xu and Zehan Dai and Pingfan Guo and Xiaocong Fu and Shanshan Liu and Lang Zhou and Wenli Tang and Tingze Feng and Meijun Chen and Li Zhan and Tianzi Wu and Erqiang Hu and Yong Jiang and Xiaochen Bo and Guangchuang Yu},
    journal = {Molecular Biology and Evolution},
    volume = {38},
    issue = {9},
    pages = {4039-4042},
    year = {2021},
    month = {06},
    issn = {0737-4038},
    doi = {10.1093/molbev/msab166},
    pmid = {34097064},
    url = {https://academic.oup.com/mbe/advance-article/doi/10.1093/molbev/msab166/6294410},
  }