Add empty = <logical> argument

NEWS.md

@@ -9,7 +9,10 @@ where the formatting is also better._
 New features:
 
 - Support for additional plot types:
-  - `type = "n"`, i.e. empty plot. (#157 @grantmcdermott)
+  - `type = "n"`, i.e. empty plot. Since `type = "n"` implicitly assumes points,
+  which limits the type of legend that can be drawn alongside the empty plot, we
+  have also added a companion `empty` argument that can be used alongside any
+  plot type. (#157, #167 @grantmcdermott)
   - `type = "boxplot"`. Simultaneously enables `plt(numeric ~ factor)`
   support, first raised in #2, so that a boxplot is automatically plotted if a
   numeric is plotted against a factor. (#154 @grantmcdermott)

R/tinyplot.R

@@ -72,12 +72,20 @@
 #'   for lines, "b" for both points and lines, "c" for empty points joined by
 #'   lines, "o" for overplotted points and lines, "s" and "S" for stair steps,
 #'   and "h" for histogram-like vertical lines. Specifying "n" produces an empty
-#'   plot over the extent of the data, but with no internal elements.
+#'   plot over the extent of the data, but with no internal elements (see also
+#'   the `empty` argument below).
 #'   - Additional tinyplot types: "boxplot" for boxplots, "density" for
 #'   densities, "polygon" or "polypath" for polygons,  "pointrange" or"errorbar"
 #'   for segment intervals, and "ribbon" or "area" for polygon intervals (where
 #'   area plots are a special case of ribbon plots with `ymin` set to 0 and
 #'   `ymax` set to `y`; see below).
+#' @param empty logical indicating whether the interior plot region should be
+#'  left empty. The default is `FALSE`. Setting to `TRUE` has a similar effect
+#'  to invoking `type = "n"` above, except that any legend artifacts owing to a
+#'  particular plot type (e.g., lines for `type = "l"` or squares for
+#'  `type = "area"`) will still be drawn correctly alongside the empty plot. In
+#'  contrast,`type = "n"` implicitly assumes a scatterplot and so any legend
+#'  will only depict points.
 #' @param xmin,xmax,ymin,ymax minimum and maximum coordinates of relevant area
 #'   or interval plot types. Only used when the `type` argument is one of
 #'   "rect" or "segments" (where all four min-max coordinates are required), or
@@ -250,7 +258,7 @@
 #'  graphics windows.
 #' @param height numeric giving the plot height in inches. Same considerations as
 #'  `width` (above) apply, e.g. will default to `tpar("file.height")` if not
-#'  specified. 
+#'  specified.
 #' @param ... other graphical parameters. See \code{\link[graphics]{par}} or
 #'   the "Details" section of \code{\link[graphics]{plot}}.
 #'
@@ -489,6 +497,7 @@ tinyplot.default = function(
     file = NULL,
     width = NULL,
     height = NULL,
+    empty = FALSE,
     ...) {
 
   dots = list(...)
@@ -1372,7 +1381,7 @@ tinyplot.default = function(
 
       # empty plot flag
       empty_plot = FALSE
-      if (type=="n" || ((length(xx)==0) && !(type %in% c("rect","segments")))) {
+      if (isTRUE(empty) || type=="n" || ((length(xx)==0) && !(type %in% c("rect","segments")))) {
         empty_plot = TRUE
       }
 

inst/tinytest/test-misc.R

@@ -6,11 +6,15 @@ f = function () {
   tinyplot(Sepal.Length ~ Petal.Length, data = iris, type = "n")
 }
 expect_snapshot_plot(f, label = "type_n")
-# empty plot(s)
 f = function () {
   tinyplot(Sepal.Length ~ Petal.Length | Species, data = iris, type = "n")
 }
 expect_snapshot_plot(f, label = "type_n_by")
+f = function () {
+  tinyplot(Sepal.Length ~ Petal.Length | Species, data = iris, type = "l", empty = TRUE)
+}
+expect_snapshot_plot(f, label = "type_l_empty")
+
 
 # log axes
 f = function() {