`bquote` didn't work with `coord_polar` + `scale_x_continuous`

[Losses]

Please notice the scale of x axis:

ggplot() +
    coord_polar() +
    scale_x_continuous(limits = c(0, 2 * pi), breaks = seq(0, 1.8, by = 0.2) * pi, labels = function(x) {
      lapply(x / pi, function(x) {
        if (x == 0) {
          return(bquote(0))
        } else {
          return(bquote(.(x) * pi))
        }
      })
    }) +
    geom_vline(xintercept = 1:6)

ggplot() +
    scale_x_continuous(limits = c(0, 2 * pi), breaks = seq(0, 1.8, by = 0.2) * pi, labels = function(x) {
      lapply(x / pi, function(x) {
        if (x == 0) {
          return(bquote(0))
        } else {
          return(bquote(.(x) * pi))
        }
      })
    }) +
    geom_vline(xintercept = 1:6)

[paleolimbot]

Thanks for opening this...it came up in one of our internal discussions recently! The issue is that the default draw_axis() handles a list() of language objects in addition to one created with expression().

ggplot2/R/guides-axis.r

Lines 89 to 96 in d1ecd03

	# break_labels can be a list() of language objects
	if (is.list(break_labels)) {
	if (any(vapply(break_labels, is.language, logical(1)))) {
	break_labels <- do.call(expression, break_labels)
	} else {
	break_labels <- unlist(break_labels)
	}
	}

should probably be its own function, and should be called to sanitize labels here:

ggplot2/R/coord-polar.r

Lines 237 to 243 in d1ecd03

	render_fg = function(self, panel_params, theme) {
	if (is.null(panel_params$theta.major)) {
	return(element_render(theme, "panel.border"))
	}
	theta <- theta_rescale(self, panel_params$theta.major, panel_params)
	labels <- panel_params$theta.labels

The workaround is to to use expression() to create labels that should be rendered using plotmath:

library(ggplot2)

label_pi <- function(x) {
  x_pi <- x / pi
  labels <- lapply(x_pi, function(x) bquote(.(x) * pi))
  labels[x_pi == 0] <- list(bquote(0))
  do.call(expression, labels)
}

p <- ggplot() +
  scale_x_continuous(
    limits = c(0, 2 * pi), 
    breaks = seq(0, 1.8, by = 0.2) * pi, 
    labels = label_pi) +
  geom_vline(xintercept = 1:6)

p + coord_polar()

^{Created on 2019-07-24 by the reprex package (v0.2.1)}

[thomasp85]

is this really a bug? Have we ever advertised using bquote for creating expressions?

[smouksassi]

bquote can be quite useful in several situation example here:
https://community.rstudio.com/t/how-to-include-grouped-parenthesis-in-ggplot-geom-text/5645

[thomasp85]

but in the example above you are not passing in a call as label, but a character vector due to the deparsing, so any use of bquote is not related to ggplot2. Also, you could achieve the same using expression and avoid the deparse call