Get all values along a given axis from a ggplot2 object (including layers).

getPlottedVals.r

# Get all variables plotted along a given axis in a ggplot.
getPlottedVals <- function(ggplotObj,axis=c("x","y"),includeLayers=TRUE) 
{
  if (is.null(axis)) {stop("In getPlotted: Argument axis not specified.")}
  getValAxis <- function(.q,axis=c("x","y")) {
    as.data.table(.q$data)[,eval(.q$mapping[[axis]])]
  }
  getLayerAxis <- function(tmpLayer,axis=c("x","y")) {
    if (!any(class(tmpLayer$data) %in% "waiver") & (any(class(tmpLayer$data)=="data.frame"))) {
      return(getValAxis(tmpLayer,axis))
    } else {return(NULL)}
  }
  res <- getValAxis(ggplotObj,axis=axis)
  if (includeLayers) {res <- c(res,unlist(lapply(ggplotObj$layer,getLayerAxis,axis=axis)))}
  return(res)
}