我正在尝试导出一张图表，用于演示，比较植物对不同肥料处理的平均响应。我正在使用R png函数导出图像，并且我需要它是下面代码中定义的大小。我唯一不知道该怎么做的是增加图例的大小，以使其看起来更大，就像图表的其余部分一样。我该怎么办？

数据：

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))

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 and export graph as png
png("scores.png",width = 1000,height = 700)

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("10","20","30","40"))+
  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))

dev.off()