R 如何编写一个将NULL传递给ggplot的函数而不产生错误

我正试图编写一个函数来创建绘图，但遇到了一个我不理解的错误。这里是一个简化的例子

可复制示例：

library (ggplot2)
# Sample data
xdata = 0:20
ydata = 20:40
dat <- data.frame(xdata, ydata)

# This works
line_p <- ggplot(dat, aes(x = xdata, y = ydata, group = NULL, color = NULL)) + geom_line()
line_p

库（ggplot2）
#样本数据
扩展数据=0:20
ydata=20:40
dat简而言之，您应该检查一下如何以编程方式创建美学映射。它允许您使用存储字符串的变量名称创建美观映射。这样，就很容易将列名作为参数传递给函数并创建相应的绘图

以下版本的函数适用于我：

# create function
line_function <- function(mydata     = dat,
                          xinput     = "x", # note the defaults are
                          yinput     = "y", # strings here
                          aes_group  = NULL,
                          aes_color  = NULL,
                          ...) {

  ggplot(mydata, # this should be the argument, not the global variable dat
         # now we create the aes binding with aes_string
         aes_string(x = xinput,
                    y = yinput,
                    group = aes_group,
                    color = aes_color)) +
    geom_line()
}

#创建函数
安德鲁，就这样，谢谢。阅读文档后最简单的更改是将lineplot行更改为：lineplot
# test the function again with explicit NULL inputs

line_test2_p <- line_function(
  mydata    = dat,
  xinput    = xdata,
  yinput    = ydata,
  aes_group = NULL,
  aes_color = NULL
)

line_test2_p

# create function
line_function <- function(mydata     = dat,
                          xinput     = "x", # note the defaults are
                          yinput     = "y", # strings here
                          aes_group  = NULL,
                          aes_color  = NULL,
                          ...) {

  ggplot(mydata, # this should be the argument, not the global variable dat
         # now we create the aes binding with aes_string
         aes_string(x = xinput,
                    y = yinput,
                    group = aes_group,
                    color = aes_color)) +
    geom_line()
}

# test the function
line_test_p <- line_function(
  mydata = dat,
  xinput = "xdata", # note the strings
  yinput = "ydata"
)

# test the function again with explicit NULL inputs
line_test2_p <- line_function(mydata    = dat,
                              xinput    = "xdata", # and strings here
                              yinput    = "ydata",
                              aes_group = NULL,
                              aes_color = NULL)