Selcted References.

[1] https://www.rstudio.com/wp-content/uploads/2015/02/rmarkdown-cheatsheet.pdf
[2] https://cran.r-project.org/web/packages/gridExtra/gridExtra.pdf
[3] https://cran.r-project.org/web/packages/ggthemes/vignettes/ggthemes.html
[4] http://www.cookbook-r.com/Graphs/Facets_(ggplot2)/
[5] https://datavizcatalogue.com/methods/density_plot.html
[6] https://www.r-bloggers.com/dplyr-example-1/
[7] http://htmlcolorcodes.com/
[8] http://waterhouse.ucdavis.edu/@@search?SearchableText=acidity