geom_violin, space between violins in one plot

General
2

You can use the width parameter.

Since you didn't provide the Seurat object cart.combined, I can't use your code, but here is a minimal reprex:

library(tidyverse)

vln_df <- tibble(Subset = rep(LETTERS[1:3], each = 100),
                 Expression = map(1:3,
                                  ~ rnbinom(100, mu = .x, size = 2)) |>
                   unlist())

vln_df |>
  ggplot(mapping = aes(x = Subset, y = Expression)) +
  geom_violin(mapping = aes(fill = Subset), scale = "width")




vln_df |>
  ggplot(mapping = aes(x = Subset, y = Expression)) +
  geom_violin(mapping = aes(fill = Subset), scale = "width", width = 0.5)

Created on 2023-06-07 with reprex v2.0.2