R ggplot2: Replacing aes_string() with aes() and .data[[varname]] when varname may be NULL

I am working on updating some older R code to make it use ggplot2’s aes() now that aes_string() is deprecated. I admit I don’t understand all of the details of tidy evaluation, but from what I do understand it seems like the replacement for aes_string(shape=shapevar) would be aes(shape=.data[[shapevar]]), or possibly aes(shape=!!sym(shapevar)).

The problem with these replacements is that they do not work when shapevar is NULL, as shown in the example below:


dat <- data.frame(x=rnorm(25),y=rnorm(25),othervar=factor(sample(1:3,25,replace=T)))

make_scatter_string <- function(plotdat,outfile,xvar="x",yvar="y",colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes_string(x=xvar,y=yvar,color=colorvar,shape=shapevar)) + geom_point()

make_scatter_data <- function(plotdat,outfile,xvar="x",yvar="y",colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes(x=.data[[xvar]],y=.data[[yvar]],color=.data[[colorvar]],shape=.data[[shapevar]])) + geom_point()

make_scatter_sym <- function(plotdat,outfile,xvar="x",yvar="y",colorvar=NULL,shapevar=NULL) {
  plt <- ggplot(plotdat,aes(x=!!sym(xvar),y=!!sym(yvar),color=!!sym(colorvar),shape=!!sym(shapevar))) + geom_point()

make_scatter_string(dat,'test1.png',colorvar="othervar") # Works, aes_string can handle shapevar being NULL
make_scatter_data(dat,'test2.png',colorvar="othervar") # Gives error message "Error in `.data[[NULL]]`: ! Must subset the data pronoun with a string, not NULL."
make_scatter_sym(dat,'test3.png',colorvar="othervar") # Gives error message "Error in `sym()`: ! Can't convert NULL to a symbol."

What is the correct way to give aes() a variable like shapevar that may contain the name of a variable in the data or may be NULL? Or is there some other value that should be used to say “don’t use this aesthetic” instead of NULL?

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