R 向xtable添加标题

我不确定这是R问题还是S问题。我在一个简单的表格（3年级和6年级）中从两个时间点查看学生的分数。我创建了表格并打印出来，但是我想添加标签，以便人们知道哪个分数来自哪个年级

我的Sweave代码是：

<<MakeData,results='asis'>>=
library(xtable)
Grade3 <- c("A","B","B","A","B","C","C","D","A","B","C","C","C","D","B","B","D","C","C","D")
Grade6 <- c("A","A","A","B","B","B","B","B","C","C","A","C","C","C","D","D","D","D","D","D")
Cohort <- table(Grade3,Grade6)
print(xtable(Cohort))
@

=
图书馆（xtable）
3级要更改
队列的标签
，请更改列名和行名：

rownames(Cohort) <- 1:4
colnames(Cohort) <- 5:8

您可以使用
caption.placement
来告诉在哪里放置标题

编辑：

根据评论，我没有回答上面的问题。要使
xtable
打印更像
table
，标签位于级别之上，请在
print.xtable
函数中使用
add.To.row
：

addtorow <- list()
addtorow$pos <- list()
addtorow$pos[[1]] <- 0
addtorow$pos[[2]] <- 0
addtorow$command <- c('& & Grade 6 & & \\\\\n', "Grade 3 & A & B & C & D \\\\\n")
print(xtable(Cohort, caption = 'My Title'), caption.placement = 'top', 
      add.to.row = addtorow, include.colnames = FALSE)

addtorow我认为OP是在pdf中寻找一些东西，比如
Grade 6
和
Grade 3
，它们包含在执行
表时打印到R控制台的内容中（Grade 3，Grade 6）
@JoshO'Brien:谢谢，我已经更新了答案，以更好地回答OPs问题。做得很好。扩展您的答案，这甚至更好：
addtorow$命令谢谢大家-Josh的行很好地清理了表格。
addtorow <- list()
addtorow$pos <- list()
addtorow$pos[[1]] <- 0
addtorow$pos[[2]] <- 0
addtorow$command <- c('& & Grade 6 & & \\\\\n', "Grade 3 & A & B & C & D \\\\\n")
print(xtable(Cohort, caption = 'My Title'), caption.placement = 'top', 
      add.to.row = addtorow, include.colnames = FALSE)