已经打开ggplot2包，输入了下面语句，plots是一片空白，我这是照着书上写的，请问为啥是空白的啊{:3_42:}
ui<-data.frame(cultivar=c("c39","c39","c39","c52","c52","c52"),data=c("d16","d20","d21","d16","d20","d21"),weight=c(3.18,2.80,2.74,2.26,3.11,1.47))
ggplot(ui,aes(x=data,y=weight,fill=cultivar))
+geom_bar(stat = "identity",width = 0.1,position=positon_dodge(0.7))