我正在尝试旋转ggplot2图的y轴标题，并将其居中于y轴。我已经能够成功旋转标题，但是即使使用hjust = 0.5命令，我也似乎无法将其定位在y轴上。旋转标题后，是否有任何想法可以使标题在y轴上居中？

dataset <- data.frame(cultivar = c('var1','var1','var3','var2','var1'),rate = c(10,20,30,40,10,40),vi = c(1354,1307,1529,1380,1416,1342,1419,1363,1282,1287,1202,1312,1294,1248,1431,1421,1439,1405,1406,1395,1540,1222,1247,1375,1235,1436,1335,1478,1356,1542,1219,1392,1524,1342))

用于生成图形的代码：

#Aggregate data frame
datasetMean <- aggregate.data.frame(dataset,by=list(dataset$cultivar,dataset$rate),mean)
datasetMean <- select(datasetMean,Group.1,Group.2,vi)
colnames(datasetMean) <- c("cultivar","rate","vi")

#Generate graph
ggplot(datasetMean,aes(x = cultivar,y = vi,fill=factor(rate)))+
  geom_bar(stat="identity",width = 0.6,position = "dodge",col = "black")+
  scale_fill_discrete(name = "Rate",labels = c("0X","1X","4X","8X"))+
  xlab("Cultivar") + ylab("vi")+
  ggtitle("VI Scores")+
  theme(plot.title = element_text(hjust = 0.5,size = 30),strip.text.x = element_text(size = 25),axis.text = element_text(size = 20),axis.title = element_text(size = 25),legend.key.size = unit(1.5,"cm"),legend.text = element_text(size = 15),legend.title = element_text(size = 20),axis.title.y = element_text(angle = 0,hjust = 0.5))