R QY数据的处理：如何定义QY因素的水平

我有一个包含Year、Quarter、QY列和许多数值变量的数据集.

#example dataset
Year = c("2019", "2020", "2021",
         "2019", "2020", "2021",
         "2019", "2020", "2021",
         "2019", "2020", "2021")
Quarter = c("1Q", "1Q", "1Q",
            "2Q", "2Q", "2Q",
            "3Q", "3Q", "3Q",
            "4Q", "4Q", "4Q")
QY = c("1Q19", "1Q20", "1Q21",
                   "2Q19", "2Q20", "2Q21",
                   "3Q19", "3Q20", "3Q21",
                   "4Q19", "4Q20", "4Q21")
VAR1 = c(10, 20, 30,
         30, 20, 25,
         27, 10, 15,
         13, 34, 25)

df <- data.frame(Year, Quarter, QY, VAR1)

我需要把所有的字符列的因素.Year和Quarter列在转换时似乎有正确的级别，但QY列没有，所以我手动定义了它.

df$Year <- as.factor(df$Year)
df$Quarter <- as.factor(df$Quarter) 
df$QY <- as.factor(df$QY) 

#check the level, QY is incorrect
sapply(df, str)

#manually defined QY level
df$QY <- factor(df$QY,
                levels = c("1Q19", "2Q19", "3Q19", "4Q19",
                           "1Q20",  "2Q20", "3Q20", "4Q20",
                           "1Q21",  "2Q21", "3Q21", "4Q21"))

有没有一种更有效的方式让R可以自动识别这些QY的水平，而不需要我手动定义它？尤其是随着QY的增长，我需要从2019年开始定义它.

我认为没有必要将Year、Quarter、QY列更改为Time Variable，但请澄清我是否应该这样做.我的计算和数据可视化需求主要是按年更改或按QY更改.