To cite package 'HH' in publications use:

Richard M. Heiberger (2014). HH: Statistical Analysis and Data Display: Heiberger and Holland. R package version 3.0-4. URL http://CRAN.R-project.org/package=HH

To cite the HH book in publications use:

Richard M. Heiberger and Burt Holland (2004). Statistical Analysis and Data Display: An Intermediate Course with Examples in S-Plus, R, and SAS. Springer-Verlag, New York. URL http://springeronline.com/0-387-40270-5

To cite likert from HH in publications use:

Richard M. Heiberger, Naomi B. Robbins (2014). Design of Diverging Stacked Bar Charts for Likert Scales and Other Applications. Journal of Statistical Software, 57(5), 1-32. URL http://www.jstatsoft.org/v57/i05/.

Corresponding BibTeX entries:

  @Manual{,
    title = {HH: Statistical Analysis and Data Display: Heiberger and
      Holland.},
    author = {Richard M. Heiberger},
    year = {2014},
    note = {R package version 3.0-4},
    url = {http://CRAN.R-project.org/package=HH},
  }
  @Book{,
    title = {Statistical Analysis and Data Display: An Intermediate
      Course with Examples in {S-Plus}, {R}, and {SAS}},
    author = {Richard M. Heiberger and Burt Holland},
    year = {2004},
    publisher = {Springer-Verlag, New York},
    edition = {First},
    url = {http://springeronline.com/0-387-40270-5},
  }
  @Article{,
    title = {Design of Diverging Stacked Bar Charts for Likert Scales
      and Other Applications},
    author = {Richard M. Heiberger and Naomi B. Robbins},
    journal = {Journal of Statistical Software},
    year = {2014},
    volume = {57},
    number = {5},
    pages = {1--32},
    url = {http://www.jstatsoft.org/v57/i05/},
  }