library(viridis) #loads the viridis color palette used in this script (must be installed the first time by install.packages("viridis"))
library(ggplot2)

#dataframe
ID <- 1:750
values <- c(rnorm(150, mean=25, sd=5), rnorm(150, mean=22, sd=3), rnorm(150, mean=30, sd=4), rnorm(150, mean=15, 3), rnorm(150, mean=40, sd=5))
groups <- c(rep("A", 150), rep("B", 150), rep("C", 150), rep("D", 150), rep("E", 150))
df <- data.frame(ID, groups, values)

#install and activate viridis
install.packages("viridis") #run this once if viridis has never been used on your machine 
library(viridis) #run every time you start R/Rstudio

#boxplot with colors and automatic legend stored in baseplot
baseplot <- ggplot(df, aes(groups, values, fill=groups)) +
              geom_boxplot() + 
              scale_fill_viridis(discrete=TRUE)
baseplot


#changing the legend position to a different side
baseplot +
  theme(legend.position="bottom")

#removing the legend
baseplot +
  theme(legend.position="none")

#putting the legend wherever you want
baseplot +
  theme(legend.position = c(0.86, 0.2))
  
#changing the legend layout to horizontal
baseplot +
  theme(legend.position = c(0.86, 0.05), legend.direction = "horizontal")

#changing the internal layout of the legend
baseplot +
  guides(fill= guide_legend(ncol = 2))

#changing the internal layout of the legend byrow
baseplot +
  guides(fill= guide_legend(ncol = 2, byrow = TRUE))


#changing the title of the legend
baseplot +
  labs(fill="New Title")

#removing the title of the legend
baseplot +
  theme(legend.title = element_blank())

#adjusting the title of the legend
baseplot +
  theme(legend.title = element_text(face = "bold", color ="red", size=11))

#changing the colors of the legend
baseplot +
  theme(legend.background = element_rect(fill = "lightblue", color = "red", size = 2))