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+#' Helper functions to use various graphics devices
+#'
+#' These helper functions generalize the use of strings (e.g.,
+#' `"svg"`, `"pdf"`) or graphic device functions (e.g.,
+#' `grDevices::svg`, `grDevices::pdf`) for in-table plots.
+#'
+#' @param filename Passed through to the graphics device.
+#' @param width,height Plot dimensions in pixels.
+#' @param res The resolution of the plot; default is 300.
+#' @param ... extra parameters passing to the graphics-device function.
+#' @param dev Character (e.g., "svg", "pdf") or function (e.g.,
+#' `grDevices::svg`, `grDevices::pdf`).
+#' @name graphics_helpers
+NULL
+
+#' @describeIn graphics_helpers Generalize 'res' and 'filename across dev functions
+#' @details
+#' - `graphics_dev` generalizes the use of 'res' and plot dimensions
+#'   across graphic devices. Raster-based devices (e.g., 'png',
+#'   'jpeg', 'tiff', 'bmp') tend to use 'res' and the width/height
+#'   units default to pixels. All other devices (e.g., 'pdf', 'svg')
+#'   tend to use inches as the default units for width/height, and
+#'   error when 'res' is provided.
+#'
+#'   The current heuristic is the look for the 'res' argument in the
+#'   function's formals; if that is present, then it is assumed that
+#'   the default units are in pixels, so 'width', 'height', and 'res'
+#'   are passed through unmodified. If 'res' is not present, then
+#'   'width' and 'height' are converted from pixels to inches, and
+#'   'res' is not passed to the function
+#'
+#'   Another purpose of this function is to generalize the different
+#'   graphic functions' use of 'file=' versus 'filename='.
+#' @return 'graphics_dev': nothing, a plot device is opened
+graphics_dev <- function(filename, width, height, res, ..., dev) {
+  dev <- match.fun(dev)
+  frmls <- names(formals(dev))
+  dots <- list(...)
+  if ("res" %in% frmls) {
+    dots <- c(dots, list(width = width, height = height, res = res))
+  } else {
+    dots <- c(dots, list(width = width / res, height = height / res))
+  }
+  filenames <- c("file", "filename")
+  found <- na.omit(match(frmls, filenames))
+  if (length(found)) {
+    dots <- c(dots, setNames(filename, filenames[ found ]))
+  } else {
+    stop("could not find a 'file' argument in graphics dev")
+  }
+  do.call(dev, dots)
+}
+
+#' @describeIn graphics_helpers Determine if plot device is svg-like
+#' @details
+#' - `is_svg` determines if the plot device is svg-like, typically one
+#'   of `"svg", `grDevices::svg`, or `svglite::svglite`
+#' @return 'is_svg': logical
+is_svg <- function(dev) {
+  if (is.character(dev)) {
+    return(grepl("svg", dev))
+  }
+  if (is.function(dev)) {
+    return(any(sapply(formals(dev), function(f) {
+      tryCatch(any(grepl("svg", as.character(f))),
+               error = function(e) FALSE)
+    })))
+  }
+  stop("unrecognized graphics 'dev': ", paste(class(dev), collapse = ","))
+}
+
+#' @describeIn graphics_helpers Determine filename extension
+#' @details
+#'
+#' - `dev_chr` determines the filename extension for the applicable
+#'   plot function; when `dev` is a string, then it is returned
+#'   unchanged; when `dev` is a function, the formals of the function
+#'   are checked for clues (i.e., default value of a `file=` argument)
+#' @return `dev_chr`: character
+#' @importFrom tools file_ext
+dev_chr <- function(dev) {
+  ext <- ""
+  if (is.character(dev)) {
+    ext <- if (dev == "svglite") "svg" else dev
+  } else if (is.function(dev)) {
+    frmls <- formals(dev)
+    filearg <- grep("^file(name)?$", names(frmls), value = TRUE)
+    if (length(filearg)) {
+      ext <- grep("\\.[[:alpha:]]+$", unlist(sapply(frmls[filearg], as.character)),
+                  value = TRUE)
+      ext <- unique(tools::file_ext(ext))[1]
+    }
+  }
+  if (is.na(ext) || !nzchar(ext)) {
+    warning("could not determine filename extension from graphic device")
+    ext <- ""
+  }
+  return(ext)
+}
+
+#' Combine file (or svg text) and parameters into a 'kableExtraInlinePlots' object
+#' 
+#' @param filename Passed through to the graphics device.
+#' @param file_ext Character, something like "png".
+#' @param dev Character (e.g., "svg", "pdf") or function (e.g.,
+#' @param width,height Plot dimensions in pixels.
+#' @param res The resolution of the plot; default is 300.
+#' @param del If the file is svg-like, then the default action is to
+#'   read the file into an embedded SVG object; once done, the file is
+#'   no longer used. The default action is to delete this file early,
+#'   set this to 'FALSE' to keep the file.
+#' @return list object, with class 'kableExtraInlinePlots'
+make_inline_plot <- function(filename, file_ext, dev,
+                             width, height, res,
+                             del = TRUE) {
+  if (is_latex() && (is_svg(file_ext) || is_svg(dev))) {
+    svg_xml <- xml2::read_xml(filename)
+    svg_text <- as.character(svg_xml)
+    if (del) {
+      unlink(filename)
+      filename <- character(0)
+    }
+  } else {
+    if (!is_latex()) {
+      filename <- paste0("file:///", normalizePath(filename, winslash = "/"))
+    }
+    svg_text <- NULL
+  }
+  out <- list(path = filename, dev = file_ext, type = "line",
+              width = width, height = height, res = res,
+              svg_text = svg_text)
+  class(out) <- c("kableExtraInlinePlots", "list")
+  return(out)
+}
+
+#' Convert arguments for a single call into Map-able args
+#'
+#' @param ... Arbitrary arguments to be possibly converted into lists
+#'   of arguments.
+#' @param lengths Allowable lengths of the arguments, typically 1 and
+#'   the length of the main variable (e.g., "x"). If 'NA' (default),
+#'   it is not enforced.
+#' @param passthru Character vector of variables to pass through with
+#'   no conversion to lists of values. Extra names (not provided in
+#'   `...`) are ignored.
+#' @param notlen1vec Character vector of variables that are known to
+#'   be length over 1 for a single plot call, so it will always be
+#'   list-ified and extra care to ensure it is grouped correctly.
+#'   Extra names (not provided in `...`) are ignored.
+#' @param notlen1lst Character vector of variables that are lists, so
+#'   the inner list length is not checked/enforced. (For example, if a
+#'   single plot call takes an argument `list(a=1,b=2,d=3)` and the
+#'   multi-data call creates three plots, then a naive match might
+#'   think that the first plot would get `list(a=1)`, second plot gets
+#'   `list(b=2)`, etc. Adding that list-argument to this 'notlen1lst'
+#'   will ensure that the full list is passed correctly.) Extra names
+#'   (not provided in `...`) are ignored.
+#' @param ignore Character vector of variables to ignore, never
+#'   returned. (Generally one can control this by not adding the
+#'   variable in the first place, but having this here allows some
+#'   sanity checks and/or programmatic usage.)
+#' @return list, generally a list of embedded lists
+listify_args <- function(..., lengths = NA,
+                         passthru = c("x", "y"),
+                         notlen1vec = c("lim", "xlim", "ylim"),
+                         notlen1lst = c("minmax", "min", "max"),
+                         ignore = c("same_lim")) {
+  indots <- list(...)
+  dotnms <- sapply(match.call(expand.dots=FALSE)$..., deparse)
+  neednames <- if (is.null(names(indots))) {
+    rep(TRUE, length(indots))
+  } else !nzchar(names(indots))
+  if (any(neednames)) {
+    names(indots)[ neednames ] <- dotnms[ neednames ]
+  }
+  dots <- indots[ intersect(names(indots), passthru) ]
+
+  # these are elements that are not typically length-1, so we need to
+  # listify slightly differently
+  nms <- intersect(names(indots), notlen1vec)
+  if (length(nms)) {
+    dots <- c(dots, Map(
+      function(L, nm) {
+        if (is.null(L)) return(list(NULL))
+        if (!is.list(L)) return(list(L))
+        if (is.na(lengths) || length(L) %in% lengths) return(L)
+        stop("length of '", nm, "' must be one of: ", paste(lengths, collapse = " or "))
+      }, indots[ nms ], nms))
+  }
+
+  # these are a little special in that the argument must be a list
+  # (regardless of its internal length)
+  nms <- intersect(names(indots), notlen1lst)
+  if (length(nms)) {
+    dots <- c(dots, Map(
+      function(L, nm) {
+        if (is.null(L)) return(list(NULL))
+        if (!length(L)) return(list(list()))
+        if (!is.list(L[[1]])) return (list(L))
+        if (is.na(lengths) || length(L) %in% lengths) return(L)
+        stop("length of '", nm, "' must be one of: ", paste(lengths, collapse = " or "))
+      }, indots[ nms ], nms))
+  }
+
+  # the remainder, those that we don't know about explicitly and are
+  # not intentionally ignored
+  nms <- setdiff(names(indots), c(passthru, notlen1vec, notlen1lst, ignore))
+  if (length(nms)) {
+    dots <- c(dots, Map(
+      function(V, nm) {
+        if (is.null(V)) return(list(NULL))
+        if (is.function(V)) return(list(V))
+        if (is.na(lengths) || length(V) %in% lengths) return(V)
+        stop("length of '", nm, "' must be one of: ", paste(lengths, collapse = " or "))
+      }, indots[ nms ], nms))
+  }
+
+  dots
+}