Use logical `NA` that can be cast to character (#82)

* use logical `NA` that can be cast to character

* chore: Use ggplot2's default color scale `na.value`

* chore: document()

* chore: Also set `na.value = "grey50"` in `scale_xaringan_discrete()`.

* chore: Add news item

---------

Co-authored-by: Garrick Aden-Buie <garrick@adenbuie.com>

NEWS.md

 # xaringanthemer (development version)
 
+- `scale_xaringan_continuous()` and `scale_xaringan_discrete()` now set
+  `na.value = "grey50"` by default for compatibility with ggplot2 > 3.5.2
+  (thanks @teunbrand, #82).
+
 # xaringanthemer 0.4.3
 
 - Update the spelling of `colour` for compatibility with ggplot2 >= 3.5.1 (@teunbrand #79).

R/ggplot2.R

   ...,
   color = NULL,
   direction = 1,
-  inverse = FALSE
+  inverse = FALSE,
+  na.value = "grey50"
 ) {
   requires_package("ggplot2", "scale_xaringan_discrete")
 
     )
   }
 
-  ggplot2::discrete_scale(aes_type, "manual", pal, ...)
+  ggplot2::discrete_scale(aes_type, "manual", pal, ..., na.value = na.value)
 }
 
 #' @rdname scale_xaringan
   color = NULL,
   begin = 0,
   end = 1,
-  inverse = FALSE
+  inverse = FALSE,
+  na.value = "grey50"
 ) {
   requires_package("ggplot2", "scale_xaringan_continuous")
   requires_package("scales", "scale_xaringan_continuous")
     palette = scales::gradient_n_pal(colors, values = NULL),
     rescaler = rescaler,
     oob = scales::censor,
+    na.value = na.value,
     ...
   )
 }

man/scale_xaringan.Rd

   ...,
   color = NULL,
   direction = 1,
-  inverse = FALSE
+  inverse = FALSE,
+  na.value = "grey50"
 )
 
 scale_xaringan_fill_discrete(..., color = NULL, direction = 1, inverse = FALSE)
   color = NULL,
   begin = 0,
   end = 1,
-  inverse = FALSE
+  inverse = FALSE,
+  na.value = "grey50"
 )
 
 scale_xaringan_fill_continuous(
 color is chosen to work well with the inverse slide styles, namely the
 value of \code{inverse_header_color}}
 
+\item{na.value}{Color to be used for missing data points.}
+
 \item{begin}{Number in the range of \code{[0, 1]} indicating to which point in the color scale the smallest data value should be mapped.}
 
 \item{end}{Number in the range of \code{[0, 1]} indicating to which point in the color scale the largest data value should be mapped.}