使用dec版本的ggplot2，我们可以传递未加引号的参数，将其转换为quosure(使用enquo)并对其求值()

With the devel version of ggplot2, we can pass unquoted arguments, convert it to quosure (with enquo) and evaluate it (!!)

ggfun <- function(dat, x.var, y.var){
  x.var <- enquo(x.var)
  y.var <- enquo(y.var)
  ggp <- ggplot(data = dat,
                aes(x = !! x.var,
                    y = !! y.var)) +
    geom_point()

  return(ggp)
}

ggfun(dat = mtcars, x.var = drat, y.var = hp)

对于带引号的字符串，请使用sym(来自rlang)将其转换为符号并进行求值

For quoted strings, convert it to symbol with sym (from rlang) and do the evaluation

ggfun <- function(dat, x.var, y.var){
  x.var <- rlang::sym(x.var)
  y.var <- rlang::sym(y.var)
  ggp <- ggplot(data = dat,
                aes(x = !! x.var,
                    y = !! y.var)) +
    geom_point()

  return(ggp)
}

ggfun(dat = mtcars, x.var = "drat", y.var = "hp")

如果要传递带引号或不带引号，则将等价更改为字符(quo_name)，然后更改为符号(sym)并求值(!!)

If we want to pass either quoted or unquoted, the quosure is changed to character (quo_name), then to symbol (sym) and evaluate (!!)

ggfun <- function(dat, x.var, y.var){
  x.var <- rlang::sym(quo_name(enquo(x.var)))
  y.var <- rlang::sym(quo_name(enquo(y.var)))
  ggp <- ggplot(data = dat,
                aes(x = !! x.var,
                    y = !! y.var)) +
    geom_point()

  return(ggp)
}

p1 <- ggfun(dat = mtcars, x.var = drat, y.var = hp)
p2 <- ggfun(dat = mtcars, x.var = "drat", y.var = "hp")

all.equal(p1, p2)
#[1] TRUE