Make the circle smaller using ggforce::geom_mark_circle

[EconKid]

I am using ggforce package, an extension of ggplot2 to label a plot.

library(tidyverse)
library(ggforce)
ggplot(iris, aes(Petal.Length, Petal.Width)) +
  geom_mark_circle(aes(color = Species,
                       label = Species),
                   label.fill = NA,
                   # 背景颜色变成透明
                   # The fill colour for the annotation box
                   label.fontface = 'plain',
                   # 避免加粗
                   expand = unit(0.2, "mm")
                   ) +
  geom_point() +
  theme(
    legend.position = 'none'
  ) +
  theme_minimal()

I want to make the circle smaller. I read the help document and try several values on the argument expand but it does works.
Does anyone know it?

[mara]

Since you're using geom_mark_circle() (as opposed to ellipse, or another shape), you're getting the smallest possible circle that can encompass all points for a specified group.

[EconKid]

HI @mara, I get your point.

ggplot(iris, aes(Petal.Length, Petal.Width)) +
  geom_mark_circle(data = iris %>% head(6),
                   aes(color = Species,
                       label = Species),
                   label.fill = NA,
                   # 背景颜色变成透明
                   # The fill colour for the annotation box
                   label.fontface = 'plain',
                   # 避免加粗
                   expand = unit(0.2, "mm")
                   ) +
  geom_point() +
  theme(
    legend.position = 'none'
  ) +
  theme_minimal()

When I change the size of the group I choose compared to the whole sample, the circle becomes automatically smaller.