Legend Operations in ggplot2

http://www.cookbook-r.com/Graphs/Legends_(ggplot2)/

Excellent blog for all operations in legend in ggplot2.


Problems when using theme() function:
http://stackoverflow.com/questions/16088981/legend-size-in-ggplot2-worldmap


Main operations of legends in ggplot2: http://zevross.com/blog/2014/08/04/beautiful-plotting-in-r-a-ggplot2-cheatsheet-3/#add-a-title-ggtitle-or-labs


An alternative way of using legends in ggplot2:
http://www.karambelkar.info/2016/03/alternative-to-using-legends-in-ggplot2/