To cite ggmap in publications, please use:

D. Kahle and H. Wickham. ggmap: Spatial Visualization with ggplot2. The R Journal, 5(1), 144-161. URL http://journal.r-project.org/archive/2013-1/kahle-wickham.pdf

Corresponding BibTeX entry:

  @Article{,
    author = {David Kahle and Hadley Wickham},
    title = {ggmap: Spatial Visualization with ggplot2},
    journal = {The R Journal},
    year = {2013},
    volume = {5},
    number = {1},
    pages = {144--161},
    url =
      {https://journal.r-project.org/archive/2013-1/kahle-wickham.pdf},
  }