using plotmath in ggrepel labels

I am trying to create a plot where I want to show all coefficients from my linear model and their respective statistical details attached at each point using ggrepel package. I have managed to create the basic plot, but what I haven't been able to figure out is how to use plotmath while creating labels. So, for example, in the plot produced below, I would like to use italics for the t-value (t) and p-value (p). Additionally, if I were to include estimates, I might also want to include the greek letter beta (β) in the label.

# loading needed libraries
library(ggrepel)
#> Loading required package: ggplot2
library(ggplot2)
library(GGally)
library(tidyverse)

# creating a dataframe containing results
(label_df <- broom::tidy(x = stats::lm(data = mtcars, wt ~ am*cyl), conf.int = TRUE) %>%
  dplyr::filter(.data = ., term != "(Intercept)") %>%
  dplyr::select(.data = ., term, estimate, conf.low, conf.high, statistic, p.value) %>%
  purrrlyr::by_row(
    .d = .,
    ..f = ~ paste(
      "t = ",
      round(.$statistic, digits = 3),
      ", p = ",
      round(.$p.value, digits = 3),
      sep = ""
    ),
    .collate = "rows",
    .to = "label",
    .labels = TRUE
  )
)
#> # tibble [3 x 7]
#>   term   estimate conf.low conf.high statistic  p.value label             
#>   <chr>     <dbl>    <dbl>     <dbl>     <dbl>    <dbl> <chr>             
#> 1 am      -0.956    -2.58      0.668    -1.21  0.238    t = -1.206, p = 0~
#> 2 cyl      0.304     0.135     0.473     3.68  0.000989 t = 3.678, p = 0.~
#> 3 am:cyl   0.0328   -0.234     0.300     0.252 0.803    t = 0.252, p = 0.~

# creating the model coefficient plot using ggcoef
plot <- GGally::ggcoef(x = stats::lm(data = mtcars, wt ~ am*cyl), exclude_intercept = TRUE)

# adding labels using ggrepel
plot +
  ggrepel::geom_label_repel(
    data = label_df,
    mapping = ggplot2::aes(x = estimate, y = term, label = label),
    size = 3,
    box.padding = grid::unit(x = 0.75, units = "lines"),
    fontface = "bold",
    direction = "y",
    color = "black",
    label.size = 0.25,
    segment.color = "black",
    segment.size = 0.5,
    segment.alpha = NULL,
    min.segment.length = 0.5,
    max.iter = 2000,
    point.padding = 0.5,
    force = 2,
    na.rm = TRUE
  )

If I use something like base::substitute or base::bquote to create the label inside purrrlyr, I get the following error:

.f must return either data frames or vectors for non-list collation

I can get rid of this error by converting it to character type but then the labels get all messed-up.

# creating a dataframe containing results
(label_df <- broom::tidy(x = stats::lm(data = mtcars, wt ~ am*cyl), conf.int = TRUE) %>%
  dplyr::filter(.data = ., term != "(Intercept)") %>%
  dplyr::select(.data = ., term, estimate, conf.low, conf.high, statistic, p.value) %>%
  purrrlyr::by_row(
    .d = .,
    ..f = ~ as.character(bquote(
      "t = "~.(round(.$statistic, digits = 3))~
        ", p = "~
        .(round(.$p.value, digits = 3))
    )),
    .collate = "rows",
    .to = "label",
    .labels = TRUE
  )
)
#> # tibble [9 x 8]
#>   term   estimate conf.low conf.high statistic  p.value  .row label       
#>   <chr>     <dbl>    <dbl>     <dbl>     <dbl>    <dbl> <int> <chr>       
#> 1 am      -0.956    -2.58      0.668    -1.21  0.238        1 ~           
#> 2 am      -0.956    -2.58      0.668    -1.21  0.238        1 "\"t = \" ~~
#> 3 am      -0.956    -2.58      0.668    -1.21  0.238        1 0.238       
#> 4 cyl      0.304     0.135     0.473     3.68  0.000989     2 ~           
#> 5 cyl      0.304     0.135     0.473     3.68  0.000989     2 "\"t = \" ~~
#> 6 cyl      0.304     0.135     0.473     3.68  0.000989     2 0.001       
#> 7 am:cyl   0.0328   -0.234     0.300     0.252 0.803        3 ~           
#> 8 am:cyl   0.0328   -0.234     0.300     0.252 0.803        3 "\"t = \" ~~
#> 9 am:cyl   0.0328   -0.234     0.300     0.252 0.803        3 0.803

Created on 2018-06-13 by the reprex package (v0.2.0).

Solution

Based on discussion in the comments, you need to use mathematical annotations correctly to avoid errors, see link.

The below label format works for me, and includes beta-estimates with the Greek symbol. list is needed to obtain commas in plotmath.

(label_df <- broom::tidy(x = stats::lm(data = mtcars, wt ~ am*cyl), conf.int = TRUE) %>%
    dplyr::filter(.data = ., term != "(Intercept)") %>%
    dplyr::select(.data = ., term, estimate, conf.low, conf.high, statistic, p.value) %>%
    purrrlyr::by_row(
      .d = .,
      ..f = ~ paste(
        "list(italic(t)==",
        round(.$statistic, digits = 3),
        ", ~italic(p)==",
        round(.$p.value, digits = 3),
        ", ~beta==",
        round(.$estimate, digits = 3),
        ")",
        sep = ""
      ),
      .collate = "rows",
      .to = "label",
      .labels = TRUE
    )
)