library(ggplot2)
library(ggthemes)
ggplot(gradel,aes(x=First, y=Math,fill=First))+geom_bar(stat="identity",width=0.5)+theme_economist()+
geom_text(aes(label=Math,vjust=-0.8,hjust=0.5,color=First),show.legend=FALSE)+ylim(min(gradel$Math,0)*1.1, max(gradel$Math)*1.1)+
labs(x="学生",y="数学")+ggtitle("学生成绩")

标题是ggtitle("学生成绩")，如何居中，感谢