在图形右侧添加第二个垂直轴的功能

Question

我在使用ggplot完成的图形的右侧添加第二个垂直轴的函数存在问题，将比例因子应用于所表示的变量。

图形和功能的代码如下：

#Libraries
library(ggplot2)    

#Data   
df <- data.frame(
      equis = c(1,1,2,2,3,3,4,4,5,5,6,6),
      ygrega = c(80,75,85,80,81,70,82,73,83,72,82,75),
      zeta = c(30,30,30,30,20,20,30,30,20,20,30,30),
      tip= c("inrix","det","inrix","det","inrix","det","inrix","det","inrix","det","inrix","det")
    )

#Graphic
grafTest <- ggplot(data= df, aes(x= equis)) +
  geom_line(aes(y= ygrega, col= tip), alpha= 0.6) +
  geom_point(aes(y= ygrega, col= tip), size= 0.6, alpha= 0.6)

#Function
afeSecAxi <- function(graf, var, facMul, namAxi, type){

  if(type=="line"){
    graf <- graf + geom_line(aes(y=var*facMul)) +
      scale_y_continuous(sec.axis = sec_axis( ~ . /facMul, name = paste(namAxi)))

    return(graf)

  }else if(type=="point"){
    graf <- graf + geom_point(aes(y=var*facMul, colour= paste(namAxi))) +
      scale_y_continuous(sec.axis = sec_axis( ~ . /facMul, name = paste(namAxi)))

    return(graf)

  }else{

    stop("Not valid type of graphic ")

  }

}

#Function example
afeSecAxi(graf = grafTest, var = df$zeta, facMul = 2.5, namAxi = "zeta", type = "line")

运行代码时，出现以下错误：

Error in FUN(X[[i]], ...) : object 'facMul' not found

我不知道为什么会出现此错误，因为如果我在函数外运行相同的代码，则图形显示正确，如下所示：

#Libraries
library(ggplot2)    

#Data 
df <- data.frame(
      equis = c(1,1,2,2,3,3,4,4,5,5,6,6),
      ygrega = c(80,75,85,80,81,70,82,73,83,72,82,75),
      zeta = c(30,30,30,30,20,20,30,30,20,20,30,30),
      tip= c("inrix","det","inrix","det","inrix","det","inrix","det","inrix","det","inrix","det")
    )

#Graphic
grafTest <- ggplot(data= df, aes(x= equis)) +
  geom_line(aes(y= ygrega, col= tip), alpha= 0.6) +
  geom_point(aes(y= ygrega, col= tip), size= 0.6, alpha= 0.6)

  graf = grafTest
  var = df$zeta
  facMul = 2.5
  namAxi = "zeta"
  type = "line"


graf <- graf + geom_line(aes(y=var*facMul)) +
              scale_y_continuous(sec.axis = sec_axis( ~ . /facMul, name = paste(namAxi)))

plot(graf)

有人知道为什么会出现错误吗？

下面显示的是初始图形和带有第二个轴的图形：

在此致以谢意和感谢。