ggtrendline - Add Trendline and Confidence Interval to 'ggplot'

Add trendline and confidence interval of linear or nonlinear regression model and show equation to 'ggplot' as simple as possible. For a general overview of the methods used in this package, see Ritz and Streibig (2008) <doi:10.1007/978-0-387-09616-2> and Greenwell and Schubert Kabban (2014) <doi:10.32614/RJ-2014-009>.