geom_rect() is drawing a box for each row in your dataframe that is why you get a dark color, since you are not mapping any variable to an aesthetic I think you should use annotate()
Thanks for the reply. But it is showing an error:
Error: geom_rect requires the following missing aesthetics: xmin
Run rlang::last_error() to see where the error occurred.
In addition: Warning message:
Ignoring unknown aesthetics: x