How to adjust font size and alignment for kableExtra table grouping headers when using pack_rows() with an index?

Question

I'm making automated reports using R Markdown and my users want the group header in their tables to be centered and to appear larger than the rest of the text. Specifically, in the following table output, I want to make Species larger and centered.

library(kableExtra)

kable(iris,escape = F)%>%
  pack_rows(index = table(fct_inorder(iris$Species)))%>%
  kable_styling(bootstrap_options = "striped", full_width = T, position = 'center')%>%
  row_spec(which(iris$Sepal.Length < 5), bold = T, background = "red")

I tried inserting formatting options within pack_rows(), but that's not how pack_rows() works so far as I can tell. As this is for rows, not columns, the options which work for add_header_above() do not work here. Plus, it's not a hardcoded header name, nor can it be.

Thanks in advance!!

Answer 1

Here are a couple of possible solutions, one each for html and pdf.

html version

html version first as this seems to be the approach you are using:

---
output: html_document
---

```{r packages, warning=FALSE, message=FALSE}

library(kableExtra)
library(forcats)
library(dplyr)

```



```{r html-answer}


iris1 <-
  iris |> 
  group_by(Species) |> 
  slice_head(n = 5)
  
kable(iris1[,-5],
      format = "html")|>
  kable_styling(bootstrap_options = "striped", 
                full_width = TRUE, 
                position = 'center') |> 
  row_spec(which(iris1$Sepal.Length < 5), 
           bold = TRUE,
           background = "red") |>
  pack_rows(index = table(fct_inorder(iris1$Species)),
            label_row_css = "font-size: 30px; text-align: center;")


```

html output (exaggerated font size for effect):

pdf version

This requires pre-formatting the group headings as LaTeX with a selected font size. I've just used typical LaTeX approach with relative font sizes. It would be possible to add exact sizes with additional LaTeX packages.

---
output: pdf_document
---

```{r pdf-answer,  warning=FALSE, message=FALSE}

library(kableExtra)
library(forcats)
library(dplyr)

iris1 <-
  iris |> 
  group_by(Species) |> 
  slice_head(n = 5) |> 
  mutate(Species = paste0(paste("\\\\begin{LARGE}", Species, "\\\\end{LARGE}")))
  
kable(iris1[,-5],
      format = "latex",
      booktabs = TRUE)|>
  row_spec(which(iris1$Sepal.Length < 5), 
           bold = TRUE,
           background = "red") |>
  pack_rows(index = table(fct_inorder(iris1$Species)),
            latex_align = "c",
            escape = FALSE)


```

pdf output: