R Language ggplot2 visualization : Set legend title （legend title） Align to ggplot2 In the middle of the legend box ( Default left alignment 、align the legend title to the middle of legend box in ggplot2)

# Simulation data

# example data from http://www.cookbook-r.com/Graphs/Legends_(ggplot2)/
df1 <- data.frame(
    sex = factor(c("Female","Female","Male","Male")),
    time = factor(c("Lunch","Dinner","Lunch","Dinner"), levels=c("Lunch","Dinner")),
    total_bill = c(13.53, 16.81, 16.24, 17.42)
)

df1

# Set legend title （legend title） Align to ggplot2 In the middle of the legend box ( Invalid case )

# example data from http://www.cookbook-r.com/Graphs/Leg