Please see the FAQ: What's a reproducible example (`reprex`) and how do I create one? Using a reprex, complete with representative data will attract quicker and more answers.
Here's an example of convex hull construction from this S/O post
library(tidyverse)
# Find the convex hull of the points being plotted
hull <- mtcars %>%
slice(chull(mpg, wt))
# Define the scatterplot
p <- ggplot(mtcars, aes(mpg, wt)) + geom_point(shape = 21)
# Overlay the convex hull
p + geom_polygon(data = hull, alpha = 0.5)
Created on 2020-03-25 by the reprex package (v0.3.0)