1. World Health Organization (WHO): https://www.who.int/
2. R for Data Science, Garrett Grolemund and Hadley Wickham, Dec 2016.
3. Ggplot2: Elegant Graphics for Data Analysis, Hadley Wickham, Reference work, 2009.
4. Italy Ministry of Health: http://www.salute.gov.it/nuovocoronavirus
5. http://www.cookbook-r.com/Graphs/