add: label_tex as an extention of ggplot2::label_parsed (#160)

DESCRIPTION

@@ -45,6 +45,7 @@ Suggests:
     sessioninfo,
     concaveman,
     deldir,
+    latex2exp,
     reshape2,
     units (>= 0.4-6),
     covr
@@ -74,6 +75,7 @@ Collate:
     'ggforce-package.R'
     'ggproto-classes.R'
     'interpolate.R'
+    'labeller.R'
     'link.R'
     'mark_circle.R'
     'mark_ellipse.R'

NAMESPACE

@@ -113,6 +113,7 @@ export(geom_spiro)
 export(geom_voronoi_segment)
 export(geom_voronoi_tile)
 export(interpolateDataFrame)
+export(label_tex)
 export(linear_trans)
 export(n_pages)
 export(position_auto)
@@ -156,6 +157,7 @@ export(trans_reverser)
 import(ggplot2)
 importFrom(MASS,fractions)
 importFrom(Rcpp,sourceCpp)
+importFrom(ggplot2,label_parsed)
 importFrom(ggplot2,layer)
 importFrom(grDevices,chull)
 importFrom(grid,addGrob)

R/labeller.R

@@ -0,0 +1,33 @@
+#' A labeller function to parse TeX syntax
+#'
+#' This function formats the strip labels of facet grids and wraps that contains
+#' TeX expressions. The latex2exp package must be installed.
+#'
+#' @seealso [ggplot2::labeller], [latex2exp::TeX()]
+#'
+#' @inheritParams ggplot2::label_parsed
+#' @inheritDotParams ggplot2::label_parsed -labels
+#'
+#' @examples
+#' # requires latex2exp package be installed
+#' if (requireNamespace("latex2exp", quietly = TRUE)) {
+#'   library(ggplot2)
+#'   d <- data.frame(x = 1, y = 1, facet = "$\\beta$")
+#'   ggplot(d, aes(x, y)) +
+#'     geom_point() +
+#'     facet_wrap(~ facet, labeller = label_tex)
+#' }
+#' @importFrom ggplot2 label_parsed
+#' @export
+label_tex <- function(labels, ...) {
+  if (!requireNamespace("latex2exp", quiet = TRUE)) {
+    stop("The latex2exp package is needed for this functionality", call. = FALSE)
+  }
+  label_parsed(
+    as.data.frame(
+      lapply(labels, latex2exp::TeX, output = "character"),
+      stringsAsFactors = FALSE
+    ),
+    ...
+  )
+}

_pkgdown.yml

@@ -110,6 +110,7 @@ reference:
       - geom_sina
       - geom_autopoint
       - geom_autodensity
+      - label_tex
       - stat_err
       - position_auto
       - position_jitternormal