R语言ggplot2可视化: 将图例标题（legend title）对齐到ggplot2中图例框的中间(默认左对齐、align legend title to middle of legend)

R语言ggplot2可视化: 将图例标题（legend title）对齐到ggplot2中图例框的中间(默认左对齐、align the legend title to the middle of legend box in ggplot2)

#仿真数据

# 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

#将图例标题（legend title）对齐到ggplot2中图例框的中间(无效的案例)

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