To cite plotKML in publications use:

Tomislav Hengl, Pierre Roudier, Dylan Beaudette, Edzer Pebesma (2015). plotKML: Scientific Visualization of Spatio-Temporal Data. Journal of Statistical Software, 63(5), 1-25. URL http://www.jstatsoft.org/v63/i05/.

Corresponding BibTeX entry:

  @Article{,
    title = {{plotKML}: Scientific Visualization of Spatio-Temporal
      Data},
    author = {Tomislav Hengl and Pierre Roudier and Dylan Beaudette and
      Edzer Pebesma},
    journal = {Journal of Statistical Software},
    year = {2015},
    volume = {63},
    number = {5},
    pages = {1--25},
    url = {http://www.jstatsoft.org/v63/i05/},
  }